There are several aspects to this commit, that don't well fit into broken down
commits, so they are detailed here:
* When a user uses response.headers['Cache-Control'] = some_value, then the
documented convention in ConditionalGet is not adhered to, in this case,
response.cache_control is ignored due to `return if
self[CACHE_CONTROL].present?`
* When a middleware sets cache-control headers that would clobber, they're
converted to symbols directly, without underscores. This would lead to bugs.
* Items that would live in :extras if set through expires_in, are placed
directly in the @cache_control hash, and not respected in many cases
(somewhat adhering to the aforementioned documentation).
* Although quite useless, any directive named 'extras' would be ignored.
The general convention applied is that expires_* take precedence, but no longer
overwrite everything and expires_* are ALWAYS applied, even if the header is
set.
I am still unhappy about the contents of this commit, and the code in general.
Ideally it should be refactored to no longer use :extras. I'd likely recommend
expanding @cache_control into a class, and giving it the power to handle the
merge in a more efficient fashion. Such a commit would be a larger change that
could have additional semantic changes for other libraries unless they utilize
expires_in in very standard ways.
