BUG/MEDIUM: mux-h2: fix encoding of non-GET/POST methods

Jerome reported that outgoing H2 failed for methods different from GET
or POST. It turns out that the HPACK encoding is performed by hand in
the outgoing headers encoding function and that the data length was not
incremented to cover the literal method value, resulting in a corrupted
HEADERS frame.

Admittedly this code should move to the generic HPACK code.

No backport is needed.

src/mux_h2.c

@@ -4096,6 +4096,7 @@ static size_t h2s_htx_bck_make_req_headers(struct h2s *h2s, struct htx *htx)
 		outbuf.area[outbuf.data++] = 0x42; // indexed name -- name=":method" (idx 2)
 		outbuf.area[outbuf.data++] = meth.len; // method length
 		memcpy(&outbuf.area[outbuf.data], meth.ptr, meth.len);
+		outbuf.data += meth.len;
 	}
 	else {
 		if (b_space_wraps(&h2c->mbuf))