added missing argument to Client.set_signature_method (self is important ;-) )

Author: brosner

oauth2/__init__.py

@@ -554,7 +554,7 @@ def __init__(self, consumer, token=None, cache=None, timeout=None,
         httplib2.Http.__init__(self, cache=cache, timeout=timeout, 
             proxy_info=proxy_info)
 
-    def set_signature_method(method):
+    def set_signature_method(self, method):
         if not isinstance(method, SignatureMethod):
             raise ValueError("Invalid signature method.")