Set a parameter in the Content-Type header. If the parameter already exists in the header, its value will be replaced with the new value. If header is Content-Type and has not yet been defined for this message, it will be set to "text/plain" and the new parameter an
(self, param, value, header='Content-Type', requote=True,
charset=None, language='')
source not stored for this graph (policy: none)