Rename Buffer toJson method to toJsonElement

[vietj]

The Buffer#toJson method is incorrectly named as it operates like toJsonObject/toJsonArray : it interprets the content of the buffer and returns a corresponding json element.

Data object toJson methods instead transform the current object into a JSON representation, for buffer that would be a base64 representation of its bytes.

This method will be renamed to toJsonElement in master and deprecated in 4.x