NotificationCenter should accept any Callable

[jasonkarns]

Summary

The NotificationCenter learns to accept any callable object as the
notification callback.

It is uncommon in Ruby to require Method instances when any Callable
will suffice. Indeed, the idiomatic object for callbacks is a block.

With this change, the notification center can accept any callable, not
only Method instances; that is, any object that responds to call
(Method, lambdas, procs, or any object with a .call method)

If the callback parameter is omitted, a block may be provided instead.

Test plan

Issues

address item 3 of #213

[jasonkarns]

Not sure what's going on with Travis. The failure is occurring before the test step even begins. https://travis-ci.org/optimizely/ruby-sdk/jobs/602833534

[brianlucas]

@jasonkarns I apologize for the delay in getting back to you on this.
This also looks good to me, and thanks for adding the test cases to accommodate the proposed implementation. One quick question - do you think it would be beneficial to provide a Proc.new {} test case as well (which also responds to :call)?

Happy to approve once rebase/remerged.

cc @mjc1283

[jasonkarns]

do you think it would be beneficial to provide a Proc.new {} test case as well (which also responds to :call)?

I can, but I don't think that additional test is valuable. We're already asserting that Proc instances can be provided (with the lambda test). It doesn't confirm anything that isn't already manifested in the tests and the lambda generation is more strict anyway (since the generated Proc checks arity).

Put another way, it's generally preferable to have only a single test fail for any given bug. Adding another test that fails for the same reason as an existing test doesn't provide value commensurate to its maintenance/carrying cost.

[brianlucas]

lgtm

[msohailhussain]

@jasonkarns can you please review this PR and merge it in this branch. It has lint fixes that should be resolved first.
jasonkarns#1

[jasonkarns]

Merged! (feel free to make any future changes like that directly on the branch. I enabled "accept edits by maintainers" to facilitate this)

[msohailhussain]

@jasonkarns Thanks.