jetty-12 ee10 ServletApiResponse.addIntHeader does not ignore headers after response committed

[janbartel]

The javadoc for HttpServletResponse.sendError says:

After using this method, the response should be considered to be committed and should not be written to.

The tck test:

com/sun/ts/tests/servlet/spec/httpservletresponse/URLClient.java.sendErrorCommitTest

does a sendError followed by setting some headers:

    PrintWriter pw = response.getWriter();
    response.sendError(HttpServletResponse.SC_OK, "sendErrorCommitTest");
    response.addIntHeader("header1", 12345);

The test expects the header not to be set, but fails because we are allowing the headers to be set after sendError.

[janbartel]

Fixed via #9668