add StatusCodeFunc for HTTP Transport implementations

[vvakame]

revise #948
add method that modify HTTP status code

This PR is works, but maybe not the best.

I want to add StatusCode(ctx context.Context, resp *graphql.Response) int but graphql.Transport doesn't limited to HTTP handling.

so, how about below API design?

type TransportConfigurator interface {
  AttachTransports (ctx context, transports []graphql.Transport)
}

and

func (a AutomaticPersistedQuery) AttachTransports (ctx context, transports []graphql.Transport) {
  for _, transport := range transports {
    httpTransport, ok := transport.(interface { HookStatusCode(f func(ctx context.Context, resp *graphql.Response) int) })
    if !ok { continue }
    httpTransport.HookStatusCode(func(ctx context.Context, resp *graphql.Response) int {
      if len(resp.Errors) == 1 && resp.Errors[0].Message == "PersistedQueryNotFound" {
        return http.StatusOK
      }
      return 0
    })
  }
}

I have:

• Added tests covering the bug / feature (see testing)
• Updated any relevant documentation (see docs)

[coveralls]

Coverage increased (+0.1%) to 63.943% when pulling 5838cbc on feat-apq-status into f869f5a on master.

[vektah]

Alternative #978, what do you think?

Even if we dont take it we should keep the integration tests.