Fauxton: fix fauxton query parameters for complex keys (COUCHDB-2249)

Fix URL generation in Fauxton when complex keys are specified as query parameters (COUCHDB-2249). 

This PR contains two changes:

1. Documents.QueryParams.parse no longer modifies it's input in-place.
2. urlParams returned by createParams contains the stringified parameters (rather than the parsed representation that it was returning previously).