Allow application/graphql content-type for clients

Expanding on https://github.com/apollo-server-integrations/apollo-server-integration-azure-functions/issues/27

[Graphql website](https://graphql.org/learn/serving-over-http/#post-request) states clients may also pass the content-type header `application/graphql`.

I will open PR to cover this use case.

cc: @shellicar