Subclass of a dict that stores values for a Cache-Control header. It
has accessors for all the cache-control directives specified in RFC 2616.
The class does not differentiate between request and response directives.
Because the cache-control directives in the HTTP header use dashes the
python descriptors use underscores for that.
To get a header of the :class:`CacheControl` object again you can convert
the object into a string or call the :meth:`to_header` method. If you plan
to subclass it and add your own items have a look at the sourcecode for
that class.
.. versionchanged:: 0.4
Setting `no_cache` or `private` to boolean `True` will set the implicit
none-value which is ``*``:
>>> cc = ResponseCacheControl()
>>> cc.no_cache = True
>>> cc
<ResponseCacheControl 'no-cache'>
>>> cc.no_cache
'*'
>>> cc.no_cache = None
>>> cc
<ResponseCacheControl ''>
In versions before 0.5 the behavior documented here affected the now
no longer existing `CacheControl` class.