code: public SendGridContentHandler register(final Class<? extends Content> clazz, final SendGridContentHandler handler) {
  		if (handler == null) {
  			throw new IllegalArgumentException("[handler] cannot be null");
  		}
  		if (clazz == null) {
  			throw new IllegalArgumentException("[clazz] cannot be null");
  		}
  
  		LOG.debug("Registering content handler {} for content type {}", handler, clazz);
  		return handlers.put(clazz, handler);
  	}
docs: Registers a handler for a given content type.
  
  @param clazz
  the content type
  @param handler
  the handler
  
  @return the handler formerly associated with the content type, if there
  was one