Define respond_to_missing? on Mocha::Mock

[tjvc]

Fixes #321.

As of Ruby 2.4, Forwardable warns when delegating to a private method.
Warnings are shown when delegating methods to a Mocha::Mock.

To determine whether to show a warning, Forwardable tests the delegated
method with the defined? operator. Mocha uses method_missing to respond
to mocked methods. defined? uses respond_to_missing? to determine
whether or not a missing method is defined. Defining respond_to_missing?
on Mocha::Mock, in place of respond_to?, prevents warnings from being
shown.

To preserve compatibility with Ruby 1.8.7, we continue to conditionally
define respond_to?. Prior to Ruby 1.9.3, defined? returns nil for method
calls handled via method_missing, so we also only test that mocked
methods are defined? in later Ruby versions.

[floehopper]

@tjvc Thanks for digging into this a bit more. There are already places in the code which do something different for older Ruby versions. Can I suggest you add require 'mocha/ruby_version' to lib/mocha/mock.rb and use if PRE_RUBY_V19 to define respond_to? with an else which defines respond_to_missing?. Ideally I'd really like to see some kind of test for this - perhaps a test could check the same thing that Forwardable is checking to decide whether or not to issue a warning...?

[tjvc]

Thanks for the pointer @floehopper. I've updated the code to conditionally define respond_to? for Ruby versions < 1.9. I've also added a test that expectations are defined? on mocks, which is the check that Forwardable uses. Because defined? doesn't seem to like method calls handled via method_missing in version 1.8.7, we only run this test for later versions. That seemed reasonable, because the behaviour is only really required for versions >= 2.4, and there are similar examples elsewhere in the code. Let me know what you think!

[floehopper]

@tjvc Thanks for updating the PR. I'll try to take a look as soon as I have time.

[tjvc]

Hi @floehopper, was just wondering if you'd had a chance to take a look at this yet?

[floehopper]

@tjvc Sorry for the delay - we've been very busy with client work. This looks great to me! I'll merge it now.

[floehopper]

I'll try to get a release out in the next few days including this change.

[tjvc]

No worries, @floehopper. Thanks for merging! 👍

[floehopper]

I've just released v1.4.0 which includes this fix.