support cache-control: immutable

[mmerickel]

response.cache_control.immutable = True and response.cache_expires(86400, immutable=True) should work.

>>> r = Response()
>>> r.cache_expires(86400, public=True, immutable=True)
>>> r.headerlist
[('Content-Type', 'text/html; charset=UTF-8'),
 ('Content-Length', '0'),
 ('Expires', 'Tue, 09 Jan 2018 17:17:03 GMT'),
 ('Cache-Control', 'max-age=86400, public')]  # expect to see immutable in the list

Currently I can only set it manually via r.headers['Cache-Control'] += ', immutable' after calling r.cache_expires(...)`.