encoding/json: support unmarshaling []int as []string with ,string tag

[chenjie4255]

Please answer these questions before submitting your issue. Thanks!

1. What version of Go are you using (go version)?
   1.7rc6
2. What operating system and processor architecture are you using (go env)?
   mac
3. What did you do?

I can marshal a int64 to json's string with a tag `json:",string"``, but it could not support a []int64 to json's string, I try the following code but it fail:

    type s struct {
        IDs []int64 `json:"ids,string"`
    }
    data := []byte(`{"ids":["123", "222"]}`)
    var ret s

    err := json.Unmarshal(data, &ret) // err here....

1. What did you expect to see?

a succeeful result.

1. What did you see instead?

a marshal error.

[odeke-em]

Duplicate of #15624, which has been marked as a feature request. Closing in favor of the mentioned issue.