Snap channel cannot be changed to 'stable'

[Barry Price]

Calling the snap.refresh() function to change a snap's channel from e.g. 'stable' to 'beta' works fine.

But attempting to change channel from anything else back to 'stable' will fail.

The refresh() function calls _refresh_store(), which extends the command to be run via _snap_args(**kw), which is the heart of the issue:

https://github.com/stub42/layer-snap/blob/master/lib/charms/layer/snap.py#L239

> if channel != 'stable':
> yield '--channel={}'.format(channel)

Removing the conditional here would solve this particular issue.

Perhaps it should be comparing the currently selected channel to the requested channel?