在interface{}上用omitempty时，和标准库行为不一致

[bobcyw]

func TestJSONITER2(t *testing.T) {
	json2 := jsoniter.ConfigCompatibleWithStandardLibrary
	a := struct {
		A interface{} `json:",omitempty" validate:"required"`
	}{A: nil}
	var jsoniterOutput bytes.Buffer
	json2.Marshal(a)
	json2.NewEncoder(&jsoniterOutput).Encode(a)
	var jsonOutput bytes.Buffer
	json.NewEncoder(&jsonOutput).Encode(a)
	assert.Equal(t, jsoniterOutput.String(), jsonOutput.String())
}

会得到

	Error Trace:	json_test.go:82
	Error:      	Not equal: 
	            	expected: "{\"A\":null}"
	            	received: "{}\n"

[taowen]

has already been fixed by: #169