http: allow folding in check_headers(), still compliant with RFC 7230 (3.2.4).

Backports: r1777460, r1777672
Submitted by: ylavic
Reviewed by: ylavic, covener, wrowe


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.2.x@1777999 13f79535-47bb-0310-9956-ffa450edef68