Cope with request.args being a dict rather than a multidict

I use ```flask.request.parameter_storage_class = werkzeug.datastructures.ImmutableDict```; that upsets ucam_webauth.flask_glue as it expects request.args to have a getlist().

Note that in the case of no request.args.getlist(), I'm omitting the check for multiple WLS-Response arguments (ImmutableDict will just give the first one).  I believe that's OK as it is still checked for validity; any attempt to pass two WLS-Responses could achieve nothing untoward.