caucase.utils: Do not wrap StreamWriter in another StreamWriter.

caucase/utils.py

@@ -507,7 +507,10 @@ def toUnicodeWritableStream(writable_stream, encoding='ascii'):
   Otherwise, returns a writable stream accepting unicode, and sending it to
   writable_stream encoded with given encoding.
   """
-  if getattr(writable_stream, 'encoding', None) is not None:
+  if (
+    isinstance(writable_stream, codecs.StreamWriter) or
+    getattr(writable_stream, 'encoding', None) is not None
+  ):
     return writable_stream
   return codecs.getwriter(encoding)(writable_stream)