Restore single-char field names inadvertantly disallowed in 2.4.25.

Backports: r1800173
PR: 61220 
Submitted by: ylavic
Reviewed by: wrowe, jchampion, ylavic



git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.4.x@1800215 13f79535-47bb-0310-9956-ffa450edef68