bug fix for sort_by. single-element list necessary.

Author: jaymedina

astroquery/mast/services.py

@@ -297,11 +297,12 @@ def service_request_async(self, service, params, page_size=None, page=None, use_
                 params_dict.setdefault(key, []).append(val)
             catalogs_request = params_dict
 
-            # Removing single-element lists. Single values will live on their own.
+            # Removing single-element lists. Single values will live on their own (except for `sort_by`)
             for key in catalogs_request.keys():
-                if len(catalogs_request[key]) == 1:
+                if (key != 'sort_by') & (len(catalogs_request[key]) == 1):
                     catalogs_request[key] = catalogs_request[key][0]
 
+        print(catalogs_request)
         response = self._request('POST', request_url, data=catalogs_request, headers=headers, use_json=use_json)
         return response