code: private List<String> getHeaderValues(String headerName) {
  		if (additionalHttpHeaders.get(headerName) == null) {
  			additionalHttpHeaders.put(headerName, new ArrayList<String>());
  		}
  		return additionalHttpHeaders.get(headerName);
  	}
docs: Gets the header values list for a given header.
  If the header doesn't have any values, an empty list will be returned.
  @param headerName the name of the header
  @return the list of values for the header