Make Marshal(RawMessage) behave like encoding/json

[greensea]

In encoding/json, there is MarshalJSON() and UnmarshaslJSON() for RawMessage. Missing this two method leads to different behavior while Marshal() jsoniter.RawMessage using std json package.

Example code below

package main
import (
    "fmt"
    "encoding/json"
    "github.com/json-iterator/go"
)

type T struct {
    Msg jsoniter.RawMessage
}

func main() {
    var j T
    
    raw := []byte(`{ "Msg": {"Foo": "Bar"}}`)
    
    jsoniter.Unmarshal(raw, &j)
        
    b1, _ := json.Marshal(j.Msg)
    fmt.Printf("%s\n", b1)                         /// Expect `{"Foo": "Bar"}` but got `"IHsiRm9vIjogIkJhciJ9"`
    
    b2, _ := jsoniter.Marshal(j.Msg)
    fmt.Printf("%s\n", b2)
}

[codecov]

Codecov Report

Merging #474 into master will decrease coverage by 0.50%.
The diff coverage is 22.22%.

@@            Coverage Diff             @@
##           master     #474      +/-   ##
==========================================
- Coverage   86.64%   86.14%   -0.51%     
==========================================
  Files          41       41              
  Lines        5100     5109       +9     
==========================================
- Hits         4419     4401      -18     
- Misses        540      566      +26     
- Partials      141      142       +1     

Impacted Files	Coverage Δ
adapter.go	81.15% <22.22%> (-8.85%)	⬇️
reflect_struct_decoder.go	82.68% <0.00%> (-2.67%)	⬇️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update a1ca083...6f96243. Read the comment docs.