revproxy: fix spelling of Trailer (from httputil)
revproxy: fix spelling of Trailer (from httputil)

Ported from Go's 2639498f.

RFC Errata 4522 (http://www.rfc-editor.org/errata_search.php?eid=4522)
notes that RFC 2616 had a typo in a list of headers that the
httputil.ReverseProxy code copied. Fix the typo in our code.

mvdan/tyk,mvdan/tyk,mvdan/tyk,lonelycode/tyk,mvdan/tyk,lonelycode/tyk,mvdan/tyk,mvdan/tyk,lonelycode/tyk,mvdan/tyk,mvdan/tyk