fixed issue with not overriding of existing value of map, and added e…

…xpect test

merge.go

@@ -53,8 +53,10 @@ func deepMerge(dst, src reflect.Value, visited map[uintptr]*visit, depth int) (e
 				if err = deepMerge(dstElement, srcElement, visited, depth+1); err != nil {
 					return
 				}
-			}
-			if !dstElement.IsValid() {
+				if !dstElement.IsValid() {
+					dst.SetMapIndex(key, srcElement)
+				}
+			default:
 				dst.SetMapIndex(key, srcElement)
 			}
 		}

mergo_test.go

@@ -136,17 +136,28 @@ func TestMaps(t *testing.T) {
 	m := map[string]simpleTest{
 		"a": simpleTest{},
 		"b": simpleTest{42},
+		"d": simpleTest{61},
 	}
 	n := map[string]simpleTest{
 		"a": simpleTest{16},
 		"b": simpleTest{},
 		"c": simpleTest{12},
+		"e": simpleTest{14},
 	}
+	expect := map[string]simpleTest{
+		"a": simpleTest{0},
+		"b": simpleTest{42},
+		"c": simpleTest{12},
+		"d": simpleTest{61},
+		"e": simpleTest{14},
+	}
+
 	if err := Merge(&m, n); err != nil {
 		t.Fatalf(err.Error())
 	}
-	if len(m) != 3 {
-		t.Fatalf(`n not merged in m properly, m must have 3 elements instead of %d`, len(m))
+
+	if !reflect.DeepEqual(m, expect) {
+		t.Fatalf("Test failed:\ngot  :\n%#v\n\nwant :\n%#v\n\n", m, expect)
 	}
 	if m["a"].Value != 0 {
 		t.Fatalf(`n merged in m because I solved non-addressable map values TODO: m["a"].Value(%d) != n["a"].Value(%d)`, m["a"].Value, n["a"].Value)