[Fix #4836] Make Rails/OutputSafety aware of safe navigation operator

[Drenmi]

The cop would not register an offense when #html_safe and #safe_concat were sent using the safe navigation operator. This change fixes that.

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Before submitting the PR make sure the following are checked:

• Wrote good commit messages.
• Commit message starts with [Fix #issue-number] (if the related issue exists).
• Used the same coding conventions as the rest of the project.
• Feature branch is up-to-date with master (if not - rebase it).
• Squashed related commits together.
• Added tests.
• Added an entry to the Changelog if the new code introduces user-observable changes. See changelog entry format.
• All tests(rake spec) are passing.
• The new code doesn't generate RuboCop offenses that are checked by rake internal_investigation.
• The PR relates to only one subject with a clear title
  and description in grammatically correct, complete sentences.
• Updated cop documentation with rake generate_cops_documentation (required only when you've added a new cop or changed the configuration/documentation of an existing cop).

[Drenmi]

@pocke: Still getting the same intermittent failure since introducing parallel tests on Travis:

NameError:
  uninitialized constant RSpec::Support::Differ
  Did you mean?  RSpec::Support::Differ

Any idea what we can do about this?

[pocke]

Sorry, I don't have any idea, I can only retry the test...

[Drenmi]

Okay, noted. Will investigate if I encounter it again. 🙂