fix for empty filters; fix for sorting of fields

2 minor fixes.
commit bb7569e checks whether there were filters set in the query and if not doesn't add them to the query url. the problem is that when looker signs the url it sorts the empty params on the top of the list so their url looks like: http://host:port/resource/?&fields=.... while the url that looker-python-sdk produces looks like http://host:port/resource?fields=...&&

commit 1bbfa0d removes the field sorting. Looker API docs are a bit confusing on this but the Ruby SDK is more specific: The fields order determines the resulting data order