fix problem with python3.6

Author: chilic

oauth2/__init__.py

@@ -490,7 +490,7 @@ def sign_request(self, signature_method, consumer, token):
             # section 4.1.1 "OAuth Consumers MUST NOT include an
             # oauth_body_hash parameter on requests with form-encoded
             # request bodies."
-            self['oauth_body_hash'] = base64.b64encode(sha1(self.body).digest())
+            self['oauth_body_hash'] = base64.b64encode(sha1(to_utf8(self.body)).digest())
 
         if 'oauth_consumer_key' not in self:
             self['oauth_consumer_key'] = consumer.key