Fix _SWITCH_UPGRADE without proposal error message

[karpetrosyan]

When we try to switch protocols when the client has not requested it, h11 throws a LocalProtocolError.

Here is how h11 do it:

h11/h11/_state.py

Lines 284 to 289 in a2c6894

	if server_switch_event not in self.pending_switch_proposals:
	raise LocalProtocolError(
	"Received server {} event without a pending proposal".format(
	server_switch_event
	)
	)

However, this exception can only be raised when the event is _SWITCH_UPGRADE, because another scenario was already checked here:

h11/h11/_connection.py

Lines 263 to 268 in a2c6894

	if type(event) is Response:
	if (
	_SWITCH_CONNECT in self._cstate.pending_switch_proposals
	and 200 <= event.status_code < 300
	):
	return _SWITCH_CONNECT

In other words, if the server_switch_event is _SWITCH_CONNECT, we always know that such an event is being proposed.