Allows for "expand" option in search requests

[zimmermanj42]

Since this utilizes NULL for the default, it should not create any BC breaks. Also, the underlying HTTP transmission code utilizes "http_build_query()" from PHP, and in this function array entries with NULL values are not included in the output. So, this change wouldn't even affect the URLs that are generated.

[zimmermanj42]

I created this PR as I was trying to get Jira issue descriptions to output as HTML. I first tried using a Markdown parser, but it looks like Jira uses it's own proprietary markup language. As such, the only way to get the HTML version of the description would be to allow for the "expand" field to be added to the API request.

For information, read the "Expansion" section on the API documentation: https://docs.atlassian.com/jira/REST/latest/

[aik099]

Please rebase and solve conflicts.