rameters for the header field, with underscores converted
to dashes.  Normally the parameter will be added as key="value" unless
value is None, in which case only the key will be added.

Example:

h.add_header('content-disposition', 'attachment', filename='bud.gif')

Note that unlike the corresponding 'email.message' method, this does
*not* handle '(charset, language, value)' tuples: all values must be
strings or None.
N