Fix Bundler/GemComment offense detection with restrictive_version_specifiers

[Drowze]

If GemComment's OnlyFor option was set to restrictive_version_specifiers, the cop would not detect an offense where it should if either:

• there was a leading space on the restrictive version argument
• the version was frozen (either by using an = operator or no operator at all)

This PR aims to fix both problems above, while also adding tests for it.

obs.: It's Hacktoberfest already! Can we have the "hacktoberfest" topic added to Rubocop repository, so pull requests may count towards the event? (as Hacktoberfest is an opt-in event since last year)

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Before submitting the PR make sure the following are checked:

• The PR relates to only one subject with a clear title and description in grammatically correct, complete sentences.
• Wrote good commit messages.
• Commit message starts with [Fix #issue-number] (if the related issue exists).
• Feature branch is up-to-date with master (if not - rebase it).
• Squashed related commits together.
• Added tests.
• Ran bundle exec rake default. It executes all tests and runs RuboCop on its own code.
• Added an entry (file) to the changelog folder named {change_type}_{change_description}.md if the new code introduces user-observable changes. See changelog entry format for details.

[koic]

obs.: It's Hacktoberfest already! Can we have the "hacktoberfest" topic added to Rubocop repository, so pull requests may count towards the event? (as Hacktoberfest is an opt-in event since last year)

Done!

[koic]

Thanks!