Force RESTClient Content-Type in mock - Groovy unit test

One of the calls I have to make in my application to another system returns JSON but with the content type as:

Content-Type: text/plain;charset=UTF-8

I have managed to get my serivce to handle this by setting the ContentType of my restClient instance

def retrieveFromRESTClientJSON(String url) throws MalformedURLException, URISyntaxException {
    def uri = validateUrl(url)
    def restClient = new RESTClient()

    log.info("REQUEST: GET request to " + uri)
    restClient.setContentType("application/json")
    def restClientResponse = restClient.get(uri: uri)
    def restClientResponseData = restClientResponse.getData()
    log.info("RESPONSE: " + restClientResponseData)

    return restClientResponseData
}

I don't know how to write a unit test for this to ensure that the methods is doing as expected and returning a json object despite the Content-Type being "text/plain"

How is it I would test this?

This is currently what I have:

void "JSON in retrieveFromRESTClientJSON's response (Content-Type text/plain) can be accessed"() {
        given: "A mocked response containing JSON as plain text"
            def JSONasText = '{"info":{"id":"1234567","version":{"foo":"stable","number": "1.2"},"device": {"location": "GB","info": "Cats are not happy on Mondays"}}}'

        def baseResponse = new BasicHttpResponse(new BasicStatusLine(HttpVersion.HTTP_1_1, 200, "OK"))
        RESTClient.metaClass.get { Map<String, ?> args ->
            def responseDecorator =  new HttpResponseDecorator(baseResponse, JSONasText) 
            responseDecorator.addHeader("Content-Type", "text/plain")
            responseDecorator.setHeader("Content-Type", "text/plain")
            return responseDecorator
        }

    when: "We call our service"
        def responseData = service.retrieveFromRESTClientJSON("http://ift.tt/1fEzkBa")

    then: "We can access all values and attributes in the returned parsed JSON"
        assert responseData.info.id == "1234567"
        assert responseData.info.version.foo == "stable"
        assert responseData.info.version.number == "1.2"
        assert responseData.info.device.location == "GB"
        assert responseData.info.device.info == "Cats are not happy on Mondays"
        GroovySystem.metaClassRegistry.removeMetaClass(RESTClient.class)
}

The reponse I get when I run this test is:

Failure:  |
JSON in retrieveFromRESTClientJSON's response (Content-Type text/plain) can be accessed(uui.RestServiceSpec)
 |
groovy.lang.MissingPropertyException: No such property: info for class: java.lang.String
    at uui.RestServiceSpec.JSON in retrieveFromRESTClientJSON's response (Content-Type text/plain) can be accessed(RestServiceSpec.groovy:189)

so it looks like the restClient.setContentType("application/json") is not working as expected and the response is being formatted as a String.