EE10 Servlet include after HttpServletResponse.getWriter().println() omits Content-Length from the response

[lorban]

Jetty version(s)
12

Description
Any EE10 servlet that does the following:

response.getWriter().println("Include:");
getServletContext().getRequestDispatcher("/test.txt").include(request, response);

will not add the Content-Length header to the response.

[gregw]

@lorban content length is not expected when include is used, instead chunking or EOF will be used.
However, we do need to write some code to covert writers to outputstreams and visa versa

[lorban]

We at least need to assert that the EE10 behavior is sane as using the OutputStream instead of the PrintWriter results in a content length header in the response, and EE9 adds content length no matter if you use the output stream or the writer.

[gregw]

@lorban See #10157 for a potential fix with new tests