fix: assign receive-loop attributes before spawning recv task

[SAY-5]

Fixes #97. Connection.__init__ spawns the receive task before assigning self._receive_timeout, so under asyncio.eager_task_factory the loop runs synchronously through asyncio.wait_for(self._reader.readline(), timeout=self._receive_timeout) and raises AttributeError, tearing the connection down before __init__ returns. Moved _observers and _receive_timeout above the _tasks.create(_receive_loop(), ...) call and added a regression test that installs the eager task factory.