[BUG] http: trailing white spaces must also be trimmed after headers

[BUG] http: trailing white spaces must also be trimmed after headers

Trailing spaces after headers were not trimmed, only the leading ones
were. An issue was detected today with a content-length value which
was padded with spaces and which was rejected. Recent updates to the
http-bis draft made it a lot more clear that such spaces must be ignored,
so this is what this patch does.

It should be backported to 1.4.