While creating these services a person really needs a simple client that they can alter the accept request-header field with. I created one using the Apache httpclient package (http://jakarta.apache.org/httpcomponents/httpclient-3.x/).
Simply download and place the commons-httpclient-*.jar file into your groovy lib directory (or somewhere you class loader will find it) and generate a Groovy program like:
def url = "URL TO SERVICE"
client = new HttpClient()
get = new GetMethod(url)
It's a simple client and one can alter the ACCEPT header value easily to check and validate REST services response to various settings of accept request-header.