'Unset a cookie with the given name (remove it from the
response).  If there are multiple cookies (e.g., two cookies
with the same name and different paths or domains), all such
cookies will be deleted.'

def unset_cookie(self, key):

    existing = self.response.headers.get_all('Set-Cookie')
    if (not existing):
        raise KeyError('No    cookies    at    all    have    been    set')
    del self.response.headers['Set-Cookie']
    found = False
    for header in existing:
        cookies = BaseCookie()
        cookies.load(header)
        if (key in cookies):
            found = True
            del cookies[key]
        header = cookies.output(header='').lstrip()
        if header:
            self.response.headers.add('Set-Cookie', header)
    if (not found):
        raise KeyError(('No    cookie    has    been    set    with    the    name    %r' % key))
