tidb_decode_key() does not return handle as an proper json array, but as a string instead

## Bug Report

Please answer these questions before submitting your issue. Thanks!

### 1. Minimal reproduce step (Required)

See [this line](https://github.com/pingcap/tidb/blob/3884b28d378ea0d8c36496258dbc7ce06d2a7234/expression/integration_test.go#L2953) in expressions/integration_test.go
```golang
result = tk.MustQuery("select tidb_decode_key( '74800000000000ffff5f7205bff199999999999a013131000000000000f9' )")
result.Check(testkit.Rows(`{"handle":"{1.1, 11}","table_id":65535}`))
```

### 2. What did you expect to see? (Required)
```json
{"handle":[1.1, 11],"table_id":65535}
```
### 3. What did you see instead (Required)
```json
{"handle":"{1.1, 11}","table_id":65535}
```
### 4. What is your TiDB version? (Required)

commit 3884b28d378ea0d8c36496258dbc7ce06d2a7234.