code: function( operation, parameters, callback ) {
  		parameters.request = operation;
  		request.get( this.baseUrl ).end( function( err, res ) {
  			if ( err ) {
  				debug( 'Error requesting %s to WMS service: %j', operation, err );
  				return callback( err );
  			}
  			// superagent set res.error for 4xx and 5xx HTTP errors
  			if ( res.error ) {
  				debug( 'HTTP error response %s', res.error.message );
  				err = new Error( res.error.message );
  				return callback( err );
  			}
  			if ( typeof callback === 'function' ) {
  				callback( err, res.text );
  			}
  		} );
  		return request;
  	}
docs: @param {String} WMS opeation (GetMap, GetCapabilities, etc)
  @param {Object} WMS request parameters
  @param {Function} callback
  - {Error} null if nothing bad happened
  - {json|Bufer} response from the server