Support clj-time Intervals/DateTime for :max-age and :expires

[KushalP]

This fixes #55.

Each commit appears with a header and body description for what I've done.

Here's a quick summary:

1. clj-time is a dependency for ring-core
2. Tests check that :max-age and :expires accept an integer (as originally outlined)
3. :max-age now accepts an Interval — this is exercised through tests
4. :expires now accepts a DateTime — exercised through tests
5. Pre-conditions make sure that only those two keys take their respective object kinds

Let me know if any further work needs to be done.

[weavejester]

Thanks for the work you've put into this so far! There are a few issues though...

The :expires attribute should not be a Unix timestamp, but in a format like "Wdy, DD Mon YYYY HH:MM:SS GMT". I believe the cookie spec gives some leeway, and as far as I can tell, the :rfc822 format type that clj-time has built in should create a cookie-compatible date string.

I think it would be something like:

(unparse (formatters :rfc822) value)

The valid-attr? function could be refactored a little by writing the cond as part of the and, e.g.

(and (contains? set-cookie-attrs key)
     (not (.contains (str value) ";")
     (case key
       :max-age (or (instance? Interval value) (integer? value))
       :expires (or (instance? DateTime value) (string? value))
       true))

Finally, I think your branch is a little out of date, as GitHub is telling me no automatic merging. You might want to rebase it off master.

[KushalP]

The commits have now been squashed into a single commit, rather than the intermediate steps.