Don't clear subscribers if channel is already private

[Rikkitp]

It fixes problem when you can't use private channels at all.
See #247 (comment) for details.

[rusikf]

I think this pr must be closed, i added description to the problem here
Short: Add to websocket-rails config

  config.keep_subscribers_when_private = true

[Envek]

No, IMHO it should be kept and merged. It covers the case when channel is already private and it should not clear existing subscribers when new one connects. Now it removes subscribers even if channel is already private and this option is just a workaround (config.keep_subscribers_when_private covers case when public channel becomes private).

Also I can see that this PR was merged in @g3d's fork here: g3d@52bc107 . @g3d may be you have any comments on this?

[g3d]

@Envek looks like the maintainers for that repo are busy, so I created own fork and merged some PRs. It works fone for us & as you can see, travis build are green (not true for ruby 2.2.x branch) for fork.

[rusikf]

@g3d Some of the contributors, like @moaa , also created another fork, which will be merged in future, as i understood.
@moaa , @DanKnox what you think about this pull request ?