getSearchHandler should return default search handler if multiple matches

The getSearchHandler method in the RestService attempts to determine which search handler to use for a given resource by comparing the parameters in the query with those supported by registered search handlers.

If no specific search handler is called for, and there are multiple candidate search handlers, an exception is thrown.

Instead, the Rest Service should return the handler with the id "default".  If none of the candidate handlers is the default, then it should throw an exception as before.