Look through interfaces and pointers when determining emptiness

When dealing with map[string]interface{}, the common case for loading
arbitrary data from yaml/json, empty strings are not overwritten because
they appear as a non-empty interface containing an empty string. This
change causes non-nil interfaces to be examined for emptiness.

mergo.go

@@ -45,7 +45,12 @@ func isEmptyValue(v reflect.Value) bool {
 		return v.Uint() == 0
 	case reflect.Float32, reflect.Float64:
 		return v.Float() == 0
-	case reflect.Interface, reflect.Ptr, reflect.Func:
+	case reflect.Interface, reflect.Ptr:
+		if v.IsNil() {
+			return true
+		}
+		return isEmptyValue(v.Elem())
+	case reflect.Func:
 		return v.IsNil()
 	case reflect.Invalid:
 		return true

pr80_test.go

@@ -0,0 +1,18 @@
+package mergo
+
+import (
+	"testing"
+)
+
+type mapInterface map[string]interface{}
+
+func TestMergeMapsEmptyString(t *testing.T) {
+	a := mapInterface{"s": ""}
+	b := mapInterface{"s": "foo"}
+	if err := Merge(&a, b); err != nil {
+		t.Fatal(err)
+	}
+	if a["s"] != "foo" {
+		t.Fatalf("b not merged in properly: a.s.Value(%s) != expected(%s)", a["s"], "foo")
+	}
+}