fix for fields scrabled and optional filters

per Looker API the resulting response will return the fields in the
order in which they were requested. The existing code sorted the fields.

per Looker API the query params should be sorted for generating the key
signature. In case where there are no filters specified the resulting
query string doesn't match the string generated by Looker because empty
query params when sorted are floting to the begining of the string. By
removing the filter param generation the url is now rendered correctly
and the python API can be used without filters