Fixed NULL pointer dereference when processing If-* headers.

Fixed NULL pointer dereference when processing If-* headers.

Previously, when processing requests with If-Match and
If-Unmodified-Since headers worker process crashed.

For example with the following code:
	try { r.return(200) }
	catch (e) { r.internalRedirect() }

The fix is to disable not_modified filter as it was done in
nginx perl module nginx/nginx@d9887ee2.