onConnectionClosed is already called inside Connection.close() when peer
is identified (via setOnClose). Calling it here would trigger an exception
as it verifies that _connection is set before un-setting it.
As a replacement, verify that _connection is not set when
Connection.close() returns (so onConnectionClosed was actually called).

git-svn-id: https://svn.erp5.org/repos/neo/trunk@2212 71dcc9de-d417-0410-9af5-da40c76e7ee4