Fix http2 transport close

transport/v2rayhttp/pool.go

@@ -0,0 +1,13 @@
+package v2rayhttp
+
+import "net/http"
+
+type ConnectionPool interface {
+	CloseIdleConnections()
+}
+
+func CloseIdleConnections(transport http.RoundTripper) {
+	if connectionPool, ok := transport.(ConnectionPool); ok {
+		connectionPool.CloseIdleConnections()
+	}
+}