Support If-[Un]Modified-Since for object HEAD

We already supported it for object GET requests, but not for
HEAD. This lets clients keep metadata up-to-date without having to
either fetch the whole object when it's changed or do their own date
parsing. They can just treat Last-Modified as opaque and update their
idea of metadata when they get a 200.

Change-Id: Iff25d8989a93d651fd2c327e1e58036e79e1bde1
Merge "Support If-[Un]Modified-Since for object HEAD"