rename hash_as_hidden_fields => render_hash_as_hidden_fields, and use it directly in _search_form
use Hash#except! to remove keys from the facet params
Refactor #params_for_search method to better match existing practice.

- remove #search_as_hidden_fields (unused)
- extract #sanitize_search_params into a stand-alone method
- change signature of #params_for_search to allow:
     - zero-arg form that just sanitized parameters
     - one-arg form that merges the request params and the arg
     - two-arg form that merges the provided source params with
           the merge params
Merge pull request #741 from projectblacklight/params_for_search

Refactor #params_for_search