Label describe() closing braces with comments to ID their starting tag

This test file is getting unwieldy, and this helps make it easier to keep
track of where you are when scrolling through the blocks
Add unit tests to identify a flaw in _paging.(next|prev)

Because calling .then (or .get, etc) on the "next" and "prev" objects
provided by the pagination parser builds a new explicit resource URL, an
error is occurring in which the search string for the first call to e.g.
`_paging.next` is overriding the search string for the subsequent call.

Example:

1. wp.posts() is requested: multiple pages exist, results get _paging.
2. results._paging.next is requested: the request object is a WPRequest
   bound directly to the full URL for the next collection page.
3. The second page of results comes back: a third page exists, so the
   results again get _paging. The _paging "next" link points to the
   third page, e.g. /wp-json/posts?page=3
4. The second page's _paging.next request is sent
5. The third page's results come back, with their own link headers
   pointing to p4 and beyond. However, since the third page's request
   was made against an endpoint with query parameters, the "search"
   parameter generated by the url module was overriding the "query"
   being set based on the "next" link to p4. This meant that for all
   subsequent requests following _paging.next, the results returned
   would be the posts for p3.
Properly merge paging link headers with the base endpoint URL

This resolves the issues (and failing unit tests) added to prevent recursive calls
to _paging.(next|prev) collections from stalling out two pages away from the start,
by explicitly copying the link header URL object search property along with the query
object.