Merge pull request #2576 from bsipocz/mast_sort_missions_list

MAINT: sorting missions list for mast for aesthetic convenience

Author: bsipocz

astroquery/mast/observations.py

@@ -88,7 +88,7 @@ def list_missions(self):
         for facet in hist_data:
             if facet['text'] == "obs_collection":
                 mission_info = facet['ExtendedProperties']['histObj']
-                missions = list(mission_info.keys())
+                missions = sorted(mission_info)
                 missions.remove('hist')
                 return missions