Add new RSpecRails/SettingsOverwritten cop

[ydah]

One problem I encountered recently was that I was directly rewriting Settings in one test, resulting in flaky tests in which other tests affected by the rewritten Settings succeeded or failed depending on the order of execution, making it difficult to determine the cause. Therefore, in this PR, I would like to propose the addition of cop to encourage the use of mocks if there are any overwriting Settings.

# bad
Settings.foo = 'bar'
Settings[:foo] = 'bar'
Settings.foo[:bar] = 'baz'
Settings.foo += 'bar'
Settings.foo << 'bar'

# good
allow(Settings).to receive(:foo).and_return('bar')
allow(Settings).to receive(:[]).with(:foo).and_return('bar')
allow(Settings.foo).to receive(:[]).with(:bar).and_return('baz')
allow(Settings).to receive(:foo).and_return(Settings.foo + "bar")

---

Before submitting the PR make sure the following are checked:

• Feature branch is up-to-date with master (if not - rebase it).
• Squashed related commits together.
• Added tests.
• Updated documentation.
• Added an entry to the CHANGELOG.md if the new code introduces user-observable changes.
• The build (bundle exec rake) passes (be sure to run this locally, since it may produce updated documentation that you will need to commit).

If you have created a new cop:

• Added the new cop to config/default.yml.
• The cop is configured as Enabled: pending in config/default.yml.
• The cop documents examples of good and bad code.
• The tests assert both that bad code is reported and that good code is not reported.
• Set VersionAdded: "<<next>>" in default/config.yml.

[Darhazer]

This can happen with any kind of object that you change, and do not restore back. Perhaps a global rspec cop ObjectNotStubbed should be implemented.

[bquorning]

I’m not sure which Settings class this is. As @Darhazer mentions, this problem applies to all globals / constants / class variables. But I don’t think we can reliably detect mutable operations. And even if we could, in most cases I would probably prefer to reset the object after testing, than to test against a stub or mock.

[ydah]

Uh. I had a big mistake. Settings are not appropriate since they are based on config gems.
https://github.com/rubyconfig/config

Also, I agree that it is hard to detect all mutable operations, so I will just close this PR.