fix __hash__() for HttpQueryError

Author: Test

graphql_server/error.py

@@ -1,3 +1,6 @@
+import json
+
+
 class HttpQueryError(Exception):
     def __init__(self, status_code, message=None, is_graphql_error=False, headers=None):
         self.status_code = status_code
@@ -13,4 +16,4 @@ def __eq__(self, other):
             other.headers == self.headers
 
     def __hash__(self):
-        return hash((self.status_code, self.message, self.headers))
+        return hash((self.status_code, self.message, json.dumps(self.headers)))