Support adding additional headers in client

[jamesabbottsmith]

What happened?

Current client implementation does not support adding additional Headers

What did you expect?

The ability to set custom Headers using the client when making a req.
This is handy for testing against GraphQL servers that have authentication.

For example

client.Header.Add("authorization", "abcd1234")`

(I've hacked it locally to make it work for me, happy to submit a PR. Should be backward compatible)

versions

• v0.9.3-dev
• go version go1.13 darwin/amd64

[vektah]

Fixed by #861 in 0.10

[harmnot]

I got issue with this
here is my client:

	clientTest := client.New(handler.NewDefaultServer(resolver.NewExecutableSchema(config)))

when I mutation query which need header

	client.MustPost(
			mutation,
			&respond,
                       client.AddHeader("Token", TOKEN_HERE)
		)

It doesn't read my header at all, any idea ? I have look your PR and implement it, no luck to me
I am using
latest gqlgen
latest go version

[lwc]

@harmnot how are you reading the header back into context? seems like you need some middleware to move the value from the http header into context?