Add more tests for WPRequest.prototype.filter, including ones covering #14
Add more tests for WPRequest.prototype.filter, including ones covering #14

Since passing a single filter as two arguments (a filter key string
and value pair) and chaining filters touch on the same code, this adds
a failing unit test to validate the behavior for the other open
filter-related issue, as well as enhancing the existing test coverage.

gerhardsletten/wordpress-rest-api,kadamwhite/wordpress-rest-api,gerhardsletten/wordpress-rest-api,brad-decker/wordpress-rest-api,brad-decker/wordpress-rest-api,kadamwhite/wordpress-rest-api,WP-API/node-wpapi,WP-API/node-wpapi,kadamwhite/wordpress-rest-api