Only store one identical language (PICARD-242).

Can happen if a recording is linked to >=2 works with the same language.
Fixup 835fa46.

picard/mbxml.py

@@ -252,7 +252,7 @@ def recording_to_metadata(node, track, config):
 def work_to_metadata(work, m, config):
     m.add("musicbrainz_workid", work.attribs['id'])
     if 'language' in work.children:
-        m.add("language", work.language[0].text)
+        m.add_unique("language", work.language[0].text)
     if 'relation_list' in work.children:
         _relations_to_metadata(work.relation_list, m, config)
 

picard/metadata.py

@@ -199,6 +199,10 @@ class Metadata(object):
         if value or value == 0:
             self._items.setdefault(name, []).append(value)
 
+    def add_unique(self, name, value):
+        if value not in self.getall(name):
+            self.add(name, value)
+
     def keys(self):
         return self._items.keys()