Check that host key exists before using in dsq_filter_rest_url

[dmatt]

Description

In environments where the $rest_url does not contain a host, like when running PHP scripts on the command line via Supervisord, php warns with a notice. To prevent these notices, this PR adds a check for the existence of 'host' before trying to use it in the dsq_filter_rest_url function.

Motivation and Context

Fixes issue #57, discussed more in #60.

How Has This Been Tested?

1. Build the docker image using these instructions https://github.com/disqus/disqus-wordpress-plugin#local-testing
2. Run bash in the docker container with docker exec -it disqus-wordpress-plugin_wordpress_1 /bin/bash
3. Create a script file my-script.php with the following

<?php
print("Hello!");
?>

4. Edit wp-config.php to remove the host:

define('WP_HOME', 'https://' );
define('WP_SITEURL', 'https:///wordpress');

5. run a script to confirm the absence of the PHP notice Undefined index: HTTP_HOST" for no web server with docker exec -it disqus-wordpress-plugin_wordpress_1 /bin/bash wp eval-file my-script.php

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
  • I have updated the documentation accordingly.
• All new and existing tests passed.

[hlgott1593]

Tested with reproduction steps