Add headers arg to GraphQLTestCase.query (#827)

* Add headers arg to GraphQLTestCase.query

* fix headers NoneType case in GraphQLTestCase.query

* Run format

Co-authored-by: Jonathan Kim <jkimbo@gmail.com>

Author: chibie

graphene_django/utils/testing.py

@@ -24,7 +24,7 @@ def setUpClass(cls):
 
         cls._client = Client()
 
-    def query(self, query, op_name=None, input_data=None, variables=None):
+    def query(self, query, op_name=None, input_data=None, variables=None, headers=None):
         """
         Args:
             query (string)    - GraphQL query to run
@@ -36,7 +36,9 @@ def query(self, query, op_name=None, input_data=None, variables=None):
                                 are provided, the ``input`` field in the ``variables``
                                 dict will be overwritten with this value.
             variables (dict)  - If provided, the "variables" field in GraphQL will be
-                                set to this value. 
+                                set to this value.
+            headers (dict)    - If provided, the headers in POST request to GRAPHQL_URL
+                                will be set to this value. 
 
         Returns:
             Response object from client
@@ -51,10 +53,17 @@ def query(self, query, op_name=None, input_data=None, variables=None):
                 body["variables"]["input"] = input_data
             else:
                 body["variables"] = {"input": input_data}
-
-        resp = self._client.post(
-            self.GRAPHQL_URL, json.dumps(body), content_type="application/json"
-        )
+        if headers:
+            resp = self._client.post(
+                self.GRAPHQL_URL,
+                json.dumps(body),
+                content_type="application/json",
+                **headers
+            )
+        else:
+            resp = self._client.post(
+                self.GRAPHQL_URL, json.dumps(body), content_type="application/json"
+            )
         return resp
 
     def assertResponseNoErrors(self, resp):