fix encoding RawMessage that contains leading space (#136)

* fix encoding RawMessage that contains leading space.

* add json/bugs/issue136

Makefile

@@ -19,7 +19,7 @@ test-json:
 	go test -cover -race ./json
 
 test-json-bugs:
-	go test -cover -race ./json/bugs/...
+	go test -race ./json/bugs/...
 
 test-json-1.17:
 	go test -cover -race -tags go1.17 ./json

json/bugs/issue136/main_test.go

@@ -0,0 +1,23 @@
+package main
+
+import (
+	"bytes"
+	"testing"
+
+	"github.com/segmentio/encoding/json"
+)
+
+func TestIssue136(t *testing.T) {
+	input := json.RawMessage(` null`)
+
+	got, err := json.Marshal(input)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	want := bytes.TrimSpace(input)
+
+	if !bytes.Equal(got, want) {
+		t.Fatalf("Marshal(%q) = %q, want %q", input, got, want)
+	}
+}

json/encode.go

@@ -858,6 +858,7 @@ func (e encoder) encodeRawMessage(b []byte, p unsafe.Pointer) ([]byte, error) {
 		s = v
 	} else {
 		var err error
+		v = skipSpaces(v) // don't assume that a RawMessage starts with a token.
 		d := decoder{}
 		s, _, _, err = d.parseValue(v)
 		if err != nil {

json/parse.go

@@ -709,7 +709,6 @@ func (d decoder) parseValue(b []byte) ([]byte, []byte, Kind, error) {
 	case '{':
 		v, b, k, err = d.parseObject(b)
 	case '[':
-		k = Array
 		v, b, k, err = d.parseArray(b)
 	case '"':
 		v, b, k, err = d.parseString(b)