Tag regression with INVITE handling

[vodik]

Caught this in production:

Traceback (most recent call last):
  File "/home/simon/src/aiosip/aiosip/application.py", line 117, in _dispatch
    await dialog.receive_message(msg)
  File "/home/simon/src/aiosip/aiosip/dialog.py", line 348, in receive_message
    self.to_details['params']['tag'] = msg.to_details['params']['tag']
  File "/home/simon/src/aiosip/aiosip/param.py", line 19, in __getitem__
    return self._param[key]
KeyError: 'tag'