MEDIUM: mux-h2: make use of hpack_encode_method() to encode the method

The HTTP method encoding was open-coded with raw HPACK bytes, which is
not suitable there. Let's make use of the new functions to avoid this.
diff --git a/src/mux_h2.c b/src/mux_h2.c
index ccf44be..4eebe1a 100644
--- a/src/mux_h2.c
+++ b/src/mux_h2.c
@@ -4062,18 +4062,7 @@
 	outbuf.data = 9;
 
 	/* encode the method, which necessarily is the first one */
-	if (outbuf.data < outbuf.size && sl->info.req.meth == HTTP_METH_GET)
-		outbuf.area[outbuf.data++] = 0x82; // indexed field : idx[02]=(":method", "GET")
-	else if (outbuf.data < outbuf.size && sl->info.req.meth == HTTP_METH_POST)
-		outbuf.area[outbuf.data++] = 0x83; // indexed field : idx[03]=(":method", "POST")
-	else if (unlikely(outbuf.data + 2 + meth.len <= outbuf.size)) {
-		/* basic encoding of the method code */
-		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 (!hpack_encode_method(&outbuf, sl->info.req.meth, meth)) {
 		if (b_space_wraps(&h2c->mbuf))
 			goto realign_again;
 		goto full;