code: public void setAttribute(String theKey, String theValue) {
  		Validate.notBlank(theKey);
  		Validate.notNull(theValue);
  		if (myAttributes == null) {
  			myAttributes = new HashMap<>();
  		}
  		myAttributes.put(theKey, theValue);
  	}
docs: Sets an attribute stored in this message.
  <p>
  Attributes are just a spot for user data of any kind to be
  added to the message for passing along the subscription processing
  pipeline (typically by interceptors). Values will be carried from the beginning to the end.
  </p>
  <p>
  Note that messages are designed to be passed into queueing systems
  and serialized as JSON. As a result, only strings are currently allowed
  as values.
  </p>
  
  @param theKey   The key (must not be null or blank)
  @param theValue The value (must not be null)