Fix skipped/incomplete core tests for 3.3 series

This pull reinstates and improves tests that were being skipped on CI as discussed in #547, in particular:

* Internally refactoring the Cookie class so tests can run regardless of headers being sent or not
* Rewriting the Cookie tests so they actually cover the setting / signing / salting logic - the original assertions were relatively meaningless.
* Rewriting the tests for Kohana::message to cover more cases and be independent of any other modules or message files
* Rewriting the tests for Feed::parse to test parsing of local sample Atom and RSS2.0 feeds.
* Removing a long-since invalid test that Response::send_headers does not send headers in CLI (it does, and has since 3.2.0).

There are now no skipped tests on kohana/core.

NB that this PR depends on the changes in #548, once that's merged I'll rebase this.

@enov are you happy that the refactorings here are all safe to release in the 3.3 bugfix series?