Rest Client Builder: 2.2.1 Doesn't send XML but does send JSON

I tried using rest builder as such with success:
def rest = new RestBuilder()
def result = rest.post(url){
contentType "application/json"
json "test"
}

However when I change it to XML as below it doesn't send the XML payload and doesn't get picked up on my API:
def rest = new RestBuilder()
def result = rest.post(url){
contentType "application/xml"
xml "test"
}

I checked send xml via curl and it works fine:
curl -i -X POST -H "Content-Type: application/xml" -d 'test' localhost:8090/TestMe/api