Groovy/Grails HTTPBuilder automatic JSON parsing

Groovy 2.4.14, Grails 2.5.6, JVM 1.8, MacOS

I'm trying (and succeeding) to use a UPS (United Parcel Service) API to obtain tracking information given a tracking number. The code works, and UPS returns a JSON string (and Content-Type: application/json). I can parse the JSON and I get exactly what I need.

However, I would kind-of prefer that HTTPBuilder do the JSON parse, because what it gives me is somewhat odd. I get back a HashMap, with the first key being the JSON body, and the value null. Like this:

{ "JSON stuff": null }

So I can parse that and get a usable structure with the normal Groovy tools:

def json = JSON.parse(data.keySet()[0])

The only thing I suspected might help was to set the "Accept" header to JSON, but that has no effect.

Here's the HTTPBuilder code:

def trackingGroup(String tracking) {
    def result;

    try {
        HTTPBuilder http = new HTTPBuilder(urlbase)
        http.request(Method.GET, ContentType.URLENC) { req ->
            uri.path = "/api/track/v1/shipment/details/${tracking}";

            requestContentType = ContentType.URLENC;
            headers['Authorization'] = "Bearer ${TOKS.getToken()}"
            headers['User-Agent'] = "Me";
            headers['Accept'] = ContentType.JSON;
            headers['transId'] = "testing"
            headers['transactionSrc'] = "testing"

            response.success = { resp, json ->
                result = json
            }
            response.failure = { resp, json ->
                result = json
            }
        }
    }
    catch (error) {
        error.printStackTrace();
        result = null
    }

    if (result == null) return "total failure";

    result = JSON.parse(result.keySet()[0])

    List alltrack = [];
    result.trackResponse.shipment.each { shipment ->
        shipment.package.each { p ->
            alltrack << p.trackingNumber
        };
    };

    return alltrack;
}

I have been able to find no good documentation anywhere about my Groovy/Grails version for the groovyx.net.http package, so what I'm asking is if there's a thing I can call in the closure parameter to the .request() call to tell it to assume JSON and parse it.

Try passing ContentType.JSON instead of ContentType.URLENC to the http.request call. According to the documentation it refers to the expected content type.