Modify tests for Last-Modified and Expires headers to enforce RFC compliance

Hi,

I've made a few changes to the tests for `lastModified()` and `expires()`:

- Change timestamps to use GMT.
- Add a test to check the format of the Last-Modified header.
- Check the full string for the Expires headers (thereby enforcing correct format)

Incidentally, running this through phpunit gave me a separate failure:

    1) ViewTest::testSetTemplatesDirectory
    Failed asserting that two strings are equal.
    --- Expected
    +++ Actual
    @@ @@
    -'templates'
    +'templates/'