Fix track filtering (#232)

* Convert Filter object to List to pass back to filter()

* Specifically check for List as string seems to now be iterable

* Ran `black .`

mopidy_gmusic/library.py

@@ -482,7 +482,7 @@ def _search_library(self, query=None, uris=None):
         result_tracks = self.tracks.values()
 
         for (field, values) in query.items():
-            if not hasattr(values, "__iter__"):
+            if not isinstance(values, list):
                 values = [values]
             # FIXME this is bound to be slow for large libraries
             for value in values:
@@ -528,21 +528,25 @@ def any_filter(track):
                     )
 
                 if field == "uri":
-                    result_tracks = filter(uri_filter, result_tracks)
+                    result_tracks = list(filter(uri_filter, result_tracks))
                 elif field == "track_name":
-                    result_tracks = filter(track_name_filter, result_tracks)
+                    result_tracks = list(
+                        filter(track_name_filter, result_tracks)
+                    )
                 elif field == "album":
-                    result_tracks = filter(album_filter, result_tracks)
+                    result_tracks = list(filter(album_filter, result_tracks))
                 elif field == "artist":
-                    result_tracks = filter(artist_filter, result_tracks)
+                    result_tracks = list(filter(artist_filter, result_tracks))
                 elif field == "albumartist":
-                    result_tracks = filter(albumartist_filter, result_tracks)
+                    result_tracks = list(
+                        filter(albumartist_filter, result_tracks)
+                    )
                 elif field == "track_no":
-                    result_tracks = filter(track_no_filter, result_tracks)
+                    result_tracks = list(filter(track_no_filter, result_tracks))
                 elif field == "date":
-                    result_tracks = filter(date_filter, result_tracks)
+                    result_tracks = list(filter(date_filter, result_tracks))
                 elif field == "any":
-                    result_tracks = filter(any_filter, result_tracks)
+                    result_tracks = list(filter(any_filter, result_tracks))
                 else:
                     raise LookupError("Invalid lookup field: %s" % field)