BUG: The 'file_name' metadata arg now overwrites 'symbol', if it is given

This is to fix the issue where 'GOOG_L's metadata lists its symbol as 'GOOG'. To resolve this, the 'file_name' is used as the canonical symbol, if it is given.

zipline/assets/assets.py

@@ -267,13 +267,9 @@ class AssetFinder(object):
             # Update the metadata object with the new sid
             self.insert_metadata(identifier=identifier, sid=kwargs['sid'])
 
-        # If the file_name is in the kwargs, it may be the symbol
+        # If the file_name is in the kwargs, it will be used as the symbol
         try:
-            file_name = kwargs.pop('file_name')
-            try:
-                kwargs['symbol']
-            except KeyError:
-                kwargs['symbol'] = file_name
+            kwargs['symbol'] = kwargs.pop('file_name')
         except KeyError:
             pass