Add a `toString` method to `JacksonJsonBuffer`

[swallez]

This will make it consistent with the behavior of JsonDataImpl, the other implementation of JsonData. Rendering as JSON in toString() comes with a cost, but provides convenience when logging plain objects or displaying them in a debugger.

[quangdutran]

Hi @swallez so this will allow client who calls toString() to see the string value that the token buffer is holding? It sounds new contributor friendly, can I try to implement it as I understand your goal as above so you could review it maybe?