Quick fixes for a couple small things checked in and installed.

Tomos' reported problem:

* broken caching with Internet Explorer (OutputPage::sendCacheControl()) vary: accept-encoding replaced with vary: user-agent to keep IE from refusing to cache pages

Related problem I noticed while fixing that:

* compressed pages from the file cache were being resent every time because IE tacked some size info on its response that broke the timestamp comparison (OutputPage::checkLastModified())

Menchi's reported problem:

* missing language links on edit previews and special pages with Nostalgia skin (SkinNostalgia::doBeforeContent())

-- brion vibber (brion @ pobox.com)