Merge r1777460, r1777672 from trunk:

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


http: follow up to r1777460.
We MUST unfold outgoing HTTP headers in any case, "message/http" is for
inner content.
Submitted by: ylavic
Reviewed by: ylavic, covener, wrowe


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.4.x@1778052 13f79535-47bb-0310-9956-ffa450edef68