= http: introduce parsing rules for invalid cookies #737

This fix introduces the ability to parse cookies headers with some valid and some invalid cookie pairs. This will allow Spray to gracefully handle (by discarding) cookies hanging around from the old Netscape style (see http://curl.haxx.se/rfc/cookie_spec.html), while still only constructing `HttpCookie`s from RFC-valid cookies.

I've made this fix using the testing style found in the existing `HttpHeaderSpec`. However, I believe more comprehensive test coverage can be accomplished, and I've put together an example in https://github.com/atlassian/spray/commit/4a376f42162a9316e0dbb04cf88489af35be38e7 ; if that looks acceptable, please let me know and I'll open a PR from the branch with those tests instead :)