encoding/json: marshal the RawMessage value type the same as its pointer type

Fixes #14493
Updates #6458 (changes its behavior)

Change-Id: I851a8113fd312dae3384e989ec2b70949dc22838
Reviewed-on: https://go-review.googlesource.com/21811
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Quentin Smith <quentin@golang.org>

api/except.txt

+pkg encoding/json, method (*RawMessage) MarshalJSON() ([]uint8, error)
 pkg net, func ListenUnixgram(string, *UnixAddr) (*UDPConn, error)
 pkg syscall (darwin-386), func Fchflags(string, int) error
 pkg syscall (darwin-386-cgo), func Fchflags(string, int) error

src/encoding/json/encode_test.go

@@ -440,7 +440,9 @@ func TestIssue6458(t *testing.T) {
 		t.Fatal(err)
 	}
 
-	if want := `{"M":"ImZvbyI="}`; string(b) != want {
+	// Until Go 1.8, this generated `{"M":"ImZvbyI="}`.
+	// See https://github.com/golang/go/issues/14493#issuecomment-255857318
+	if want := `{"M":"foo"}`; string(b) != want {
 		t.Errorf("Marshal(x) = %#q; want %#q", b, want)
 	}
 }
@@ -717,3 +719,14 @@ func TestMarshalFloat(t *testing.T) {
 	test(0, 32)
 	test(math.Copysign(0, -1), 32)
 }
+
+func TestMarshalRawMessageValue(t *testing.T) {
+	const val = "\"some value\""
+	b, err := Marshal(RawMessage(val))
+	if err != nil {
+		t.Fatal(err)
+	}
+	if string(b) != val {
+		t.Errorf("got %q; want %q", b, val)
+	}
+}

src/encoding/json/stream.go

@@ -246,9 +246,9 @@ func (enc *Encoder) SetEscapeHTML(on bool) {
 // be used to delay JSON decoding or precompute a JSON encoding.
 type RawMessage []byte
 
-// MarshalJSON returns *m as the JSON encoding of m.
-func (m *RawMessage) MarshalJSON() ([]byte, error) {
-	return *m, nil
+// MarshalJSON returns m as the JSON encoding of m.
+func (m RawMessage) MarshalJSON() ([]byte, error) {
+	return m, nil
 }
 
 // UnmarshalJSON sets *m to a copy of data.