[v2.01] added special handling for updating 'content-type' header

noticed while testing the following rule in the response data set:

[
{
    "url": /^.*$/,
    "headers": function(){
		// normalize media types using the new "+json" suffix
		// examples:
		//   * http://www.w3.org/TR/json-ld/#application-ld-json
		//   * http://json-ld.org/contexts/person.jsonld

		var $headers = {};
		var pattern = /\+json$/i;
		if (pattern.test(response.content_type)){
			$headers['content-type'] = 'application/json';
			$headers['content-disposition'] = null;
			response.content_type = $headers['content-type'];
		}
		return $headers;
    }
}
]

..that an Exception was being logged (with the debug flag set).

* turns out that the API treats this header differently
* rather than calling the normal `setResponseHeader` method,
  a special attribute needs to be set: `contentType`

now, the above rule works beautifully.