Issue Parsing x-www-form-urlencoded POST Requests

[antsankov]

I was working on an endpoint that needed to receive requests in the format x-www-form-urlencoded. This causes issues with the JSON decoder line in handler.go:

data, err := decodePayload(payloadString(r.Body))
        if err != nil {
            // Payload decoding failed.
            ctx = ctx.setError(BadRequest(err.Error()))
        }

Obviously if the data isn't in JSON format it can't be decoded. However if you even attempt to decode r.Body, because it is a io stream, it is emptied and can't be accessed.

I devised a way of fixing this in my project, and was curious if there was any interest in a pull request for this issue.

[tylertreat-wf]

Yeah, ideally payload decoding could be pluggable based on the Content-Type. Feel free to open a PR.