HttpGraphQlClientTransport should respect an existing, explicitly set Content-Type

[busches]

With 1.0.0-RC1, the Client now sends the Content-Type header as application/graphql+json

We're trying to override the headers here, but it has no affect:

HttpGraphQlClient.builder(webClient)
        .url(url)
        .headers { header -> header.contentType = MediaType.APPLICATION_JSON }
        .build()

graphQlClient.documentName(documentName)
            .variables(variables)
            .retrieve("myQuery")
            .toEntityList(myQuery::class.java)

It's still coming over as application/graphql+json.

We need to do this, as the the Apollo graphql gateway we're using does not accept the graphql header.
Error: POST body missing, invalid Content-Type, or JSON object has no keys. (it's not the other two).

[giger85]

Overriding the content-type value (actually application/json) of request header is needed for me, too.

[rstoyanchev]

Currently HttpGraphQlTransport sets the content type unconditionally. We can update that to check if it's already been set.