Fix Rails/FilePath to detect offenses from complex string interpolation

[r7kamura]

So far, this cop has only detected offenses from dstr nodes that end with str node, but I think there are offenses that would be missed if it did so. Therefore, I have changed the #on_dstr implementation to detect offenses even in examples such as the one I have added as a test case.

---

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.
• If this is a new cop, consider making a corresponding update to the Rails Style Guide.