Arguments not in correct order return no data

[chriscarrollsmith]

Calling imf_dataset with freq=["A", "Q", "M"] returns no data, apparently because the API expects arguments in the order they appear in the params dataframe (alphabetized by description).

Thus, we should probably sort arguments before making the API call.

Also make sure the list is being generated in the correct order when freq is omitted, because I had some issues making an API call without freq.