Allow Zero Length Packet (ZLP) to be sent

[HiFiPhile]

From USB 2.0 specification:

 A bulk transfer is complete when the endpoint does one of the following:
• Has transferred exactly the amount of data expected
• Transfers a packet with a payload size less than wMaxPacketSize or transfers a zero-length packet
When a bulk transfer is complete, the Host Controller retires the current IRP and advances to the next IRP.
If a data payload is received that is larger than expected, all pending bulk IRPs for that endpoint will be
aborted/retired.

If a ZLP is not sent, the device layer could wait for more data.

A workaround for Windows is in pyserialwin32.py:

    def write(self, data):
        """Output the given byte string over the serial port."""
        if not self.is_open:
            raise PortNotOpenError()
        #~ if not isinstance(data, (bytes, bytearray)):
            #~ raise TypeError('expected %s or bytearray, got %s' % (bytes, type(data)))
        # convert data (needed in case of memoryview instance: Py 3.1 io lib), ctypes doesn't like memoryview
        data = to_bytes(data)
        #~ win32event.ResetEvent(self._overlapped_write.hEvent)
        n = win32.DWORD()
        if data:
            success = win32.WriteFile(self._port_handle, data, len(data), ctypes.byref(n), self._overlapped_write)
        else:
            success = win32.WriteFile(self._port_handle, 0, 0, ctypes.byref(n), self._overlapped_write)
        if self._write_timeout != 0:  # if blocking (None) or w/ write timeout (>0)
            if not success and win32.GetLastError() not in (win32.ERROR_SUCCESS, win32.ERROR_IO_PENDING):
                raise SerialException("WriteFile failed ({!r})".format(ctypes.WinError()))

            # Wait for the write to complete.
            #~ win32.WaitForSingleObject(self._overlapped_write.hEvent, win32.INFINITE)
            win32.GetOverlappedResult(self._port_handle, self._overlapped_write, ctypes.byref(n), True)
            if win32.GetLastError() == win32.ERROR_OPERATION_ABORTED:
                return n.value  # canceled IO is no error
            if n.value != len(data):
                raise SerialTimeoutException('Write timeout')
            return n.value
        else:
            errorcode = win32.ERROR_SUCCESS if success else win32.GetLastError()
            if errorcode in (win32.ERROR_INVALID_USER_BUFFER, win32.ERROR_NOT_ENOUGH_MEMORY,
                             win32.ERROR_OPERATION_ABORTED):
                return 0
            elif errorcode in (win32.ERROR_SUCCESS, win32.ERROR_IO_PENDING):
                # no info on true length provided by OS function in async mode
                return len(data)
            else:
                raise SerialException("WriteFile failed ({!r})".format(ctypes.WinError()))