mod_negotiation: LanguagePriority tags must be treated case-insensitive,

per https://tools.ietf.org/html/rfc7231#section-3.1.3.1  

PR: 39730
Backports: r1850989
Submitted by: jailletc36
Reviewed by: jailletc36, wrowe, jim



git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.4.x@1851468 13f79535-47bb-0310-9956-ffa450edef68