[#546] Allow GET API actions to not have parameters

For actions like package_list or group_list you still had to provide
parameters as in:

http://demo.ckan.org/api/3/action/package_list?foo

These changes allow to not pass any param so an empty dict is passed to
the logic layer, where if a necessary param is missing a ValidationError
will be raised.

Also adds some missing tests for GETable actions.
Merge pull request #546 from okfn/546-get-actions-no-params

GETable actions always need a URL parameter even if not required