Accept event-stream for POST for urql-graphql

[StevenACoffman]

Fix #3275

There was an attempt to support SSE, but it broke HTTP POST.

#3153 added the following to the HTTP POST transport:

gqlgen/graphql/handler/transport/http_post.go

Lines 30 to 32 in 4157ef9

	if strings.Contains(r.Header.Get("Accept"), "text/event-stream") {
	return false
	}

From a general HTTP point of view though, the Accept header is there to communicate which content types the client can understand, and indeed clients like javascript's urql advertise that they can understand SSE, even when the server is only set up to support POST. The presence of text/event-stream in the client's accept header shouldn't mean that the request can't be served by HTTP POST.

For instance, the javascript client urql, has this header hardcoded for every request:

https://github.com/urql-graphql/urql/blob/b9f34fd19ee5f9022db4d2f9eb610943c787dac1/packages/core/src/internal/fetchOptions.ts#L149-L154

const headers: HeadersInit = {
    accept:
      operation.kind === 'subscription'
        ? 'text/event-stream, multipart/mixed'
        : 'application/graphql-response+json, application/graphql+json, application/json, text/event-stream, multipart/mixed',
  };

Signed-off-by: Steve Coffman steve@khanacademy.org

[coveralls]

coverage: 56.727% (-0.01%) from 56.737%
when pulling 63a4e0c on accept_event_stream_POST
into 4157ef9 on master.