Fix GeneratorFile for working with BufferedReader

The user files system uses a BufferedReader along with the magic library to determine the mime type of the user file being served. Currently, BufferedReader fails with an exception on Swift storage, because Swift storage returns a GeneratorFile, which is missing the `readable()` method.

util/registry/generatorfile.py

@@ -33,6 +33,9 @@ class GeneratorFile(object):
       raise StopIteration
     return r
 
+  def readable(self):
+    return not self._closed
+
   def readline(self):
     buf = []
     while True: