Prevent infinite recursion when GSSErrors occurs

When calling log.exception you do not need to pass an exception object,
it will fetch it from the stack automatically. Passing the exception
object in results in infinite recursion.

      File ".../lib/python2.6/site-packages/requests_kerberos/kerberos_.py", line 117, in generate_request_header
        log.exception(e)
      File "/usr/lib64/python2.6/logging/__init__.py", line 1088, in exception
        self.error(*((msg,) + args), **{'exc_info': 1})
      File "/usr/lib64/python2.6/logging/__init__.py", line 1082, in error
        self._log(ERROR, msg, args, **kwargs)
      File "/usr/lib64/python2.6/logging/__init__.py", line 1173, in _log
        self.handle(record)
      File "/usr/lib64/python2.6/logging/__init__.py", line 1183, in handle
        self.callHandlers(record)
      File "/usr/lib64/python2.6/logging/__init__.py", line 1220, in callHandlers
        hdlr.handle(record)
      File "/usr/lib64/python2.6/logging/__init__.py", line 679, in handle
        self.emit(record)
      File "/usr/lib64/python2.6/logging/__init__.py", line 804, in emit
        self.handleError(record)
      File "/usr/lib64/python2.6/logging/__init__.py", line 733, in handleError
        traceback.print_exception(ei[0], ei[1], ei[2], None, sys.stderr)
      File "/usr/lib64/python2.6/traceback.py", line 126, in print_exception
        lines = format_exception_only(etype, value)
      File "/usr/lib64/python2.6/traceback.py", line 179, in format_exception_only
        return [_format_final_exc_line(stype, value)]
      File "/usr/lib64/python2.6/traceback.py", line 205, in _format_final_exc_line
        valuestr = _some_str(value)
      File "/usr/lib64/python2.6/traceback.py", line 214, in _some_str
        return str(value)
    RuntimeError: maximum recursion depth exceeded while getting the str of an object
Merge pull request #40 from borgstrom/prevent-recursion

Prevent infinite recursion when GSSErrors occurs