This PR is fix an incorrect autocorrect for Rails/FilePath when File.join with Rails.root and path starting with /.

Auto-correct of following code would change the path.

Code to reproduce:

File.join(Rails.root, '/app/models', 'user.rb')

Expected behavior:

Rails.root.join("app/models/user.rb").to_s
# => "/rails/root/path/app/models/user.rb"

Actual behavior:

Rails.root.join("/app/models/user.rb").to_s
# => "/app/models/user.rb"

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.