Fixed http transport headers

gql/transport/http.py

@@ -1,5 +1,5 @@
 class HTTPTransport(object):
 
-    def __init__(self, url, client_headers=None):
+    def __init__(self, url, headers=None):
         self.url = url
-        self.client_headers = None
+        self.headers = headers

gql/transport/requests.py

@@ -20,7 +20,7 @@ def execute(self, document, variable_values=None):
                 'query': query_str,
                 'variables': variable_values
             },
-            headers=self.client_headers,
+            headers=self.headers,
             auth=self.auth
         )
         request.raise_for_status()