Encoder that encodes a single object as a UTF-8 encoded JSON string.
This encoder works equivalently to first converting the object to a JSON string, and then UTF-8 encoding the string, but without creating an intermediate string.
stream. [...]
                  object into UTF-8 encoded JSON.
                  Converter<RS, RT> view of this stream transformer. [...]
                  this with other. [...]
                  _defaultBufferSize