The form data sent to the server should be application/x-www-form-urlencoded
This replaces spaces with '+' before being RFC 1738 encoded, so a literal
'+' must be %-encoded also.
See https://cygwin.com/ml/cygwin/2014-01/msg00287.html et seq.
Signed-off-by: Jon Turney <jon.turney@dronecode.org.uk>