The HeaderDict class is accessible via the paste.response module.
Also there is an .add(key, value) method, which sets the key, or adds the value to the current value (turning it into a list if necessary).
For passing to WSGI there is a .headeritems() method which is like .items() but unpacks value that are lists. It also handles encoding -- all headers are encoded in ASCII (if they are unicode).
@@: Should that encoding be ISO-8859-1 or UTF-8? I'm not sure what the spec says.
See the source for more information.