Add ability to set headers in GraphQLTestCases

[curlywurlycraig]

When calling self.query within tests, there is currently no way to specify headers. It would be helpful to be able to specify headers in cases where, e.g., GraphQLView is overridden to check the Authorization header.

I've identified the line that needs to be updated:

graphene-django/graphene_django/utils/testing.py

Line 56 in 254e59c

self.GRAPHQL_URL, json.dumps(body), content_type="application/json"