http: handle trailer headers in all chunked responses

HTTP allows that a server sends trailing headers after all the chunks
have been sent WITHOUT signalling their presence in the first response
headers. The "Trailer:" header is only a SHOULD there and as we need to
handle the situation even without that header I made libcurl ignore
Trailer: completely.

Test case 1116 was added to verify this and to make sure we handle more
than one trailer header properly.

Reported by: Patrick McManus
Bug: http://curl.haxx.se/bug/view.cgi?id=3052450