Add FromDrop func

drops.go

@@ -4,3 +4,14 @@ package liquid
 type Drop interface {
 	ToLiquid() interface{}
 }
+
+// FromDrop returns returns object.ToLiquid() if object's type implement this function;
+// else the object itself.
+func FromDrop(object interface{}) interface{} {
+	switch object := object.(type) {
+	case Drop:
+		return object.ToLiquid()
+	default:
+		return object
+	}
+}