Set 'Vary: X-Request-With' to disable payload caching

Reverts fb683c65cb5430f46099a6d845feb594c28f3cf4. Using the header is better solution as proposed by @hrach. Related thread is https://bugzilla.mozilla.org/show_bug.cgi?id=633743. This fix should be removed when header will be set by Nette Framework natively.
typo