##### 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 username for use in HTTP basic authentication.
      - This parameter can be used without C(url_password) for sites that allow empty passwords.
      - Since version 2.8 you can also use the C(username) alias for this option.
    type: str
    aliases: ['username']
    version_added: '1.6'
  url_password:
    description:
        - The password for use in HTTP basic authentication.
        - If the C(url_username) parameter is not specified, the C(url_password) parameter will not be used.
        - Since version 2.8 you can also use the 'password' alias for this option.
    type: str
    aliases: ['password']
    version_added: '1.6'
  force_basic_auth:
    description:
      - Force the sending of the Basic authentication header upon initial request.
      - httplib2, the library used by the uri module only sends authentication information when a webservice
        responds to an initial request with a 401 status. Since some basic auth services do not properly
        send a 401, logins will fail.
    type: bool
    default: no
    version_added: '2.0'
  client_cert:
    description:
      - PEM formatted certificate chain file to be used for SSL client authentication.
      - This file can also include the key as well, and if the key is included, C(client_key) is not required.
    type: path
    version_added: '2.4'
  client_key: