Header Operations

Working with the headers in a message involves knowing how to use the methods on the HeaderSet and on the individual Headers within the HeaderSet.

1. Adding new Headers
   New headers can be added to the HeaderSet by using one of the provided add..Header() methods.
2. Retrieving Headers
   Headers are retrieved through the HeaderSet's get() and getAll() methods.
3. Check if a Header Exists
   You can check if a named header is present in a HeaderSet by calling its has() method.
4. Removing Headers
   Removing a Header from the HeaderSet is done by calling the HeaderSet's remove() or removeAll() methods.
5. Modifying a Header's Content
   To change a Header's content you should know what type of header it is and then call it's appropriate setter method. All headers also have a setFieldBodyModel() method that accepts a mixed parameter and delegates to the correct setter.