no longer allow an ETag header in 304 responses.

PR: 19355
Submitted by: Geoffrey Young <geoff apache.org>
Reviewed by: Jeff Trawick, Thom May


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/APACHE_2_0_BRANCH@101840 13f79535-47bb-0310-9956-ffa450edef68