TUN-5836: QUIC transport no longer sets body to nil in any condition

Setting the body to nil was rendering cloudflared to crashing with
a SIGSEGV in the odd case where the hostname accessed maps to a
TCP origin (e.g. SSH/RDP/...) but the eyeball sends a plain HTTP
request that does not go through cloudflared access (thus not wrapped
in websocket as it should).

Instead, QUIC transport now sets http.noBody in that condition, which
deals with the situation gracefully.

connection/quic.go

@@ -342,7 +342,7 @@ func buildHTTPRequest(connectRequest *quicpogs.ConnectRequest, body io.ReadClose
 	//   * there is no transfer-encoding=chunked already set.
 	// So, if transfer cannot be chunked and content length is 0, we dont set a request body.
 	if !isWebsocket && !isTransferEncodingChunked(req) && req.ContentLength == 0 {
-		req.Body = nil
+		req.Body = http.NoBody
 	}
 	stripWebsocketUpgradeHeader(req)
 	return req, err

connection/quic_test.go

@@ -345,7 +345,7 @@ func TestBuildHTTPRequest(t *testing.T) {
 				},
 				ContentLength: 0,
 				Host:          "cf.host",
-				Body:          nil,
+				Body:          http.NoBody,
 			},
 			body: io.NopCloser(&bytes.Buffer{}),
 		},