'Adds a (name, value) pair, doesn\'t overwrite the value if it already
exists.
>>> headers = HTTPHeaderDict(foo=\'bar\')
>>> headers.add(\'Foo\', \'baz\')
>>> headers[\'foo\']
\'bar, baz\''

def add(self, key, value):

    self._data.setdefault(key.lower(), []).append((key, value))
