Honor charm channel in bundles

Fixes #496

Author: johnsca

juju/bundle.py

@@ -436,7 +436,7 @@ async def run(self, context):
 
         entity_id = await context.charmstore.entityId(self.charm)
         log.debug('Adding %s', entity_id)
-        await context.client_facade.AddCharm(channel=None, url=entity_id, force=False)
+        await context.client_facade.AddCharm(channel=self.channel, url=entity_id, force=False)
         return entity_id
 
     def __str__(self):