##### 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'"
}
```

- If C(dest) is a directory, either the server provided filename or, if
        none provided, the base name of the URL on the remote server will be
        used. If a directory, C(force) has no effect.
      - If C(dest) is a directory, the file will always be downloaded
        (regardless of the C(force) option), but replaced only if the contents changed..
    type: path
    required: true
  tmp_dest:
    description:
      - Absolute path of where temporary file is downloaded to.
      - When run on Ansible 2.5 or greater, path defaults to ansible's remote_tmp setting
      - When run on Ansible prior to 2.5, it defaults to C(TMPDIR), C(TEMP) or C(TMP) env variables or a platform specific value.
      - U(https://docs.python.org/2/library/tempfile.html#tempfile.tempdir)
    type: path
    version_added: '2.1'
  force:
    description:
      - If C(yes) and C(dest) is not a directory, will download the file every
        time and replace the file if the contents change. If C(no), the file
        will only be downloaded if the destination does not exist. Generally
        should be C(yes) only for small local files.
      - Prior to 0.6, this module behaved as if C(yes) was the default.
      - Alias C(thirsty) has been deprecated and will be removed in 2.13.
    type: bool
    default: no
    aliases: [ thirsty ]
    version_added: '0.7'
  backup:
    description:
      - Create a backup file including the timestamp information so you can get