MEDIUM: mux-h2: pass trailers to H1 (legacy mode)

When forwarding an H2 request to an H1 server, if the request doesn't
have a content-length header field, it is chunked. In this case it is
possible to send trailers to the server, which is what this patch does.
If the transfer is performed without chunking, then the trailers are
silently discarded.

src/mux_h2.c

@@ -3469,7 +3469,12 @@ next_frame:
 		 * data block message emit the trailing CRLF */
 		if (!b_putblk(rxbuf, "0\r\n", 3))
 			goto fail;
-		/* FIXME: emit the decoded trailers here */
+
+		outlen = h2_make_h1_trailers(list, b_tail(rxbuf), try);
+		if (outlen > 0)
+			b_add(rxbuf, outlen);
+		else
+			goto fail;
 	}
 
 	goto done;