http: strip trailing OWS from header values

HTTP header values can have trailing OWS, but it should be stripped.  It
is not semantically part of the header's value, and if treated as part
of the value, it can cause spurious inequality between expected and
actual header values.

Note that a single SPC of leading OWS is common before the field-value,
and it is already handled by the HTTP parser by stripping all leading
OWS. It is only the trailing OWS that must be stripped by the parser
user.

	header-field   = field-name ":" OWS field-value OWS
	    ; https://tools.ietf.org/html/rfc7230#section-3.2
	OWS            = *( SP / HTAB )
	    ; https://tools.ietf.org/html/rfc7230#section-3.2.3

Fixes: https://hackerone.com/reports/730779

PR-URL: https://github.com/nodejs-private/node-private/pull/191


Reviewed-By: Matteo <Collina &lt;matteo.collina@gmail.com>
Reviewed-By: Ben <Noordhuis &lt;info@bnoordhuis.nl>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Beth Griggs <Bethany.Griggs@uk.ibm.com>