Allow OPTIONS request for CORS-enabled servers

[pkosiec]

Hello @vektah, firstly I have to say that this library is awesome!

I've run into an issue with CORS enabled server using gqlgen. All OPTIONS requests from client fails (Bad request -{“data”:null,“errors”:[{“message”:“json body could not be decoded: EOF”}]}), because currently only GET and POST requests are supported.

		if r.Method == "GET" {
			...
		} else {

// Assumes that this is POST request

			if err := json.NewDecoder(r.Body).Decode(&reqParams); err != nil {
				sendErrorf(w, http.StatusBadRequest, "json body could not be decoded: "+err.Error())
				return
			}
		}

Can you please add OPTIONS request support?
Thank you very much in advance!

[vektah]

Why not use a piece of dedicated middleware for this? I'm quite partial to https://github.com/rs/cors

[pkosiec]

I'm already using it... But I see I probably screwed this up by enabling OptionsPassthrough option in this cors library... I have to confirm this. Anyway thank you for quick response! I will let you know what I found.

[pkosiec]

@vektah Yup, that was my fault. I've accidentally set OptionsPassthrough to true. Thank you with solving my issue and sorry for bothering you!
Also, thanks for polishing the code in your newly created PR! You rock, man!