fix(selector): set global do not work (#2489)

* fix(selector): set global do not work

* fix(selector): test

* fix(selector): return early

* fix(selector): use struct

* fix(selector): use struct

selector/global.go

@@ -1,13 +1,21 @@
 package selector
 
-var globalSelector Builder
+var globalSelector = &wrapSelector{}
+
+var _ Builder = (*wrapSelector)(nil)
+
+// wrapSelector wrapped Selector, help override global Selector implementation.
+type wrapSelector struct{ Builder }
 
 // GlobalSelector returns global selector builder.
 func GlobalSelector() Builder {
-	return globalSelector
+	if globalSelector.Builder != nil {
+		return globalSelector
+	}
+	return nil
 }
 
 // SetGlobalSelector set global selector builder.
 func SetGlobalSelector(builder Builder) {
-	globalSelector = builder
+	globalSelector.Builder = builder
 }