Fix some bugs in the CRDT resource.

* Context is no longer emitted when an empty binary.
* Fix argument order to lists:member/2 in riak_kv_web:is_crdt_type/1.
* Moved malformed_request/2 above forbidden/2 to reflect that it is
  called first.
* Moved POST body check into process_post/2, out of
  malformed_request/2. Halt with 400 when the body is malformed.
* Ensure that the bucket type is checked when security is undefined.
* Fix arguments to riak_kv_crdt functions in produce_json/2 and ensure
  we encode the response body.
* Fix order of arguments to riak_kv_wm_utils:maybe_decode_uri/2 in
  path_segment_to_bin/2.