This reverts commit 638d4cb.

Include feed type="" attr in feed_link

Fixes #830

… dev

Swap apache_request_headers() for $_SERVER

Thanks @makss

New implementation supports returning feed deltas (just the new entries)
if the A-IM plus either the If-Modified-Since or If-None-Match headers.

* Always send a Vary header for feeds so caches will be variant aware.
* Disable caching when Txp is not in production.
* Don't lie about last modification dates.
* Derive ETags from last modification timestamps.

Fix feed delta implementation.

JSON value for prerelease (beta, etc) should be `null` if there is no beta, and version number if if there is one.

Add JSON for release and prerelease versions

Fixes #574

Thanks @phiw13