##### SUMMARY
<!--- Explain the problem briefly below -->
get_url does not support check mode (`--check`) when using the checksum format `<algorithm>:<url>`.

##### ISSUE TYPE
- Bug Report

##### COMPONENT NAME
- get_url

##### ANSIBLE VERSION
<!--- Paste verbatim output from "ansible --version" between quotes -->
```paste below
ansible 2.8.4
  config file = /etc/ansible/ansible.cfg
  configured module search path = [u'/home/nbe/.ansible/plugins/modules', u'/usr/share/ansible/plugins/modules']
  ansible python module location = /home/nbe/.local/lib/python2.7/site-packages/ansible
  executable location = /home/nbe/.local/bin/ansible
  python version = 2.7.15+ (default, Nov 27 2018, 23:36:35) [GCC 7.3.0]
```

##### CONFIGURATION
<!--- Paste verbatim output from "ansible-config dump --only-changed" between quotes -->
```paste below

```

##### OS / ENVIRONMENT
<!--- Provide all relevant information below, e.g. target OS versions, network device firmware, etc. -->
- Ubuntu 18.04.3 LTS

##### STEPS TO REPRODUCE
<!--- Describe exactly how to reproduce the problem, using a minimal test-case -->
Run playbook in check mode: `ansible-playbook --check site.yml`
<!--- Paste example playbooks or commands between quotes below -->
```yaml
---
- hosts: localhost
  connection: local

  vars:
    vault_version: '1.2.2'

  tasks:

    - name: Download zip archive using checksum
      get_url:
        url: 'https://releases.hashicorp.com/vault/{{ vault_version }}/vault_{{ vault_version }}_linux_amd64.zip'
        dest: '/tmp/vault_{{ vault_version }}_linux_amd64.zip'
        checksum: 'sha256:7725b35d9ca8be3668abe63481f0731ca4730509419b4eb29fa0b0baa4798458'

    - name: Download zip archive using checksum url
      get_url:
        url: 'https://releases.hashicorp.com/vault/{{ vault_version }}/vault_{{ vault_version }}_linux_amd64.zip'
        dest: '/tmp/vault_{{ vault_version }}_linux_amd64_2.zip'
        checksum: 'sha256:https://releases.hashicorp.com/vault/{{ vault_version }}/vault_{{ vault_version }}_SHA256SUMS'
```

<!--- HINT: You can paste gist.github.com links for larger files -->

##### EXPECTED RESULTS
<!--- Describe what you expected to happen when running the steps above -->
The second task `'Download zip archive using checksum url'` using the format `<algorithm>:<url>` must not fail and should show the same result as the first task (ok/changed) in check mode.

##### ACTUAL RESULTS
<!--- Describe what actually happened. If possible run with extra verbosity (-vvvv) -->
The second task `'Download zip archive using checksum url'` using the format `<algorithm>:<url>` fails in check mode:
<!--- Paste verbatim command output between quotes -->
```paste below
fatal: [localhost]: FAILED! => {
    "changed": false, 
    "invocation": {
        "module_args": {
            "attributes": null, 
            "backup": null, 
            "checksum": "sha256:https://releases.hashicorp.com/vault/1.2.2/vault_1.2.2_SHA256SUMS", 
            "client_cert": null, 
            "client_key": null, 
            "content": null, 
            "delimiter": null, 
            "dest": "/tmp/vault_1.2.2_linux_amd64.zip", 
            "directory_mode": null, 
            "follow": false, 
            "force": false, 
            "force_basic_auth": false, 
            "group": null, 
            "headers": null, 
            "http_agent": "ansible-httpget", 
            "mode": null, 
            "owner": null, 
            "regexp": null, 
            "remote_src": null, 
            "selevel": null, 
            "serole": null, 
            "setype": null, 
            "seuser": null, 
            "sha256sum": "", 
            "src": null, 
            "timeout": 10, 
            "tmp_dest": null, 
            "unsafe_writes": null, 
            "url": "https://releases.hashicorp.com/vault/1.2.2/vault_1.2.2_linux_amd64.zip", 
            "url_password": null, 
            "url_username": null, 
            "use_proxy": true, 
            "validate_certs": true
        }
    }, 
    "msg": "Unable to find a checksum for file 'vault_1.2.2_linux_amd64.zip' in 'https://releases.hashicorp.com/vault/1.2.2/vault_1.2.2_SHA256SUMS'"
}
```

the original file back if you somehow clobbered it incorrectly.
    type: bool
    default: no
    version_added: '2.1'
  sha256sum:
    description:
      - If a SHA-256 checksum is passed to this parameter, the digest of the
        destination file will be calculated after it is downloaded to ensure
        its integrity and verify that the transfer completed successfully.
        This option is deprecated and will be removed in version 2.14. Use
        option C(checksum) instead.
    default: ''
    type: str
    version_added: "1.3"
  checksum:
    description:
      - 'If a checksum is passed to this parameter, the digest of the
        destination file will be calculated after it is downloaded to ensure
        its integrity and verify that the transfer completed successfully.
        Format: <algorithm>:<checksum|url>, e.g. checksum="sha256:D98291AC[...]B6DC7B97",
        checksum="sha256:http://example.com/path/sha256sum.txt"'
      - If you worry about portability, only the sha1 algorithm is available
        on all platforms and python versions.
      - The third party hashlib library can be installed for access to additional algorithms.
      - Additionally, if a checksum is passed to this parameter, and the file exist under
        the C(dest) location, the I(destination_checksum) would be calculated, and if
        checksum equals I(destination_checksum), the file download would be skipped
        (unless C(force) is true). If the checksum does not equal I(destination_checksum),
        the destination file is deleted.
    type: str