when looking at StringRequestTarget I found the following things unnecessary
(1) create a Charset object where a String is sufficient for the encoding
(2) Write into a stream first, then read back from it and write to the response stream using an internal buffer
(3) flush the output stream
(4) having to specify the charset in the 'contentType' and again in the 'charset' parameter.
I made up an own version of StringRequestTarget and attached a patch for it.