Add get/set methods to remaining main query classes (comments and terms)

[Offereins]

Interacting with the $query_vars property in WP_Query or WP_User_Query is handled through the get() and set() methods. It would be great if this could be a unified generic interface for WP's other main query classes, WP_Comment_Query and WP_Term_Query.

Patches attached.

[Offereins]

Comment Query methods

[Offereins]

Term Query methods

[birgire]

Default as null instead of empty string for $default.

[birgire]

Added tests

[birgire]

Default as null instead of empty string for $default

[birgire]

Added tests

[birgire]

I agree with @Offereins here, I think it's good to have a get/set methods consistency in the query classes.

Above I attached the Tests_Comment_Query::test_get_and_set() and Tests_Term_Query::test_get_and_set() tests based on the Tests_User_Query::test_get_and_set() test.

I also updated the patches, based on the WP_User_Query::get() discussion here:

https://core.trac.wordpress.org/ticket/21426

regarding the null vs empty string return value for non-existing query variables.

[birgire]

The 39120.diff​ patch:

• combines all the above patches + tests into a single patch.
• removes @access public from the docBlocks.
• Adds comments to tests.
• Adjustments according to the Coding Standard.