Fix channel._on_close() without a method frame.

If a channel close is triggered by connection close which doesn't have a
method_frame (eg, because of a socket disconnection due to missed
heartbeats), channel._on_close() will be called with a method_frame of
None.  This patch handles this, to avoid looking up attributes on None.
Merge pull request #276 from rboulton/channel_onclose_fix

Fix channel._on_close() without a method frame.