Ticket #820 (defect)

According to PEP 333, start_response with exc_info should not raise exception if no output was sent yet

http://www.python.org/dev/peps/pep-0333/#error-handling

"If no output has been written when an exception occurs, the call to start_response will return normally, and the application will return an error body to be sent to the browser. However, if any output has already been sent to the browser, start_response will reraise the provided exception."

CherryPy WSGI server raises the exception for every start_response with exc_info except for the first one.

The patch is trivial: add "if self.sent_headers:" around "raise exc_inf0[0], exc_info[1], exc_info[2]"