fix ability to change password to a non-ascii one

https://github.com/vector-im/riot-web/issues/2658

synapse/handlers/auth.py

@@ -656,8 +656,8 @@ class AuthHandler(BaseHandler):
             Whether self.hash(password) == stored_hash (bool).
         """
         if stored_hash:
-            return bcrypt.hashpw(password + self.hs.config.password_pepper,
-                                 stored_hash.encode('utf-8')) == stored_hash
+            return bcrypt.hashpw(password.encode('utf8') + self.hs.config.password_pepper,
+                                 stored_hash.encode('utf8')) == stored_hash
         else:
             return False