Fix buffer issue when response buffering is active and a short results

[stephanr]

While working on #1618 I found this issue, that when response buffering is active, a single record CSV output was suppressed.

This was boiling down to the point that a Writer was created in HttpResponseBuffer but not flushed when flushBuffer() is called like it is done for the xmlWriter.

To work around the issue of a possible closed writer instead of flush(), which raises an exception on an already closed stream, the checkError() was used which only flushes if the writer is still open (see [1]).

References:

1. https://docs.oracle.com/en/java/javase/11/docs/api/java.base/java/io/PrintWriter.html#checkError()