goprotobuf: Fix proto.Merge handling of scalar bytes fields.

Previously the []byte would be treated as a repeated field,
and the new data would be appended. However, []byte is a
scalar field and the old data should be replaced instead.

Fixes #51.

LGTM=r
R=r
CC=golang-codereviews
https://codereview.appspot.com/91820043

proto/clone.go

@@ -131,8 +131,11 @@ func mergeAny(out, in reflect.Value) {
 		}
 		switch in.Type().Elem().Kind() {
 		case reflect.Bool, reflect.Float32, reflect.Float64, reflect.Int32, reflect.Int64,
-			reflect.String, reflect.Uint32, reflect.Uint64, reflect.Uint8:
+			reflect.String, reflect.Uint32, reflect.Uint64:
 			out.Set(reflect.AppendSlice(out, in))
+		case reflect.Uint8:
+			// []byte is a scalar bytes field.
+			out.Set(in)
 		default:
 			for i := 0; i < n; i++ {
 				x := reflect.Indirect(reflect.New(in.Type().Elem()))

proto/clone_test.go

@@ -167,6 +167,12 @@ var mergeTests = []struct {
 			RepBytes: [][]byte{[]byte("sham"), []byte("wow")},
 		},
 	},
+	// Check that a scalar bytes field replaces rather than appends.
+	{
+		src:  &pb.OtherMessage{Value: []byte("foo")},
+		dst:  &pb.OtherMessage{Value: []byte("bar")},
+		want: &pb.OtherMessage{Value: []byte("foo")},
+	},
 }
 
 func TestMerge(t *testing.T) {