[Fix #1071] Partially fix Rails/FilePath autocorrect for root filepaths by marking cop as unsafe for autocorrection

[corsonknowles]

We correctly add a @safety annotation for Rails/FilePath since it can produce invalid code like this:

# pass any string argument beginning with '/' to Rails.root.join and it will omit the Rails path

(dev)> Rails.root.join '/'
=> #<Pathname:/>
(dev)> Rails.root.join '//'
=> #<Pathname://>
(dev)> Rails.root.join '/abc'
=> #<Pathname:/abc>
(dev)> Rails.root.join '/abc/'
=> #<Pathname:/abc/>

See also:

• FilePath safe autocorrect breaks code #1071
• Rails/FilePath autocorrect shouldn't remove important trailing slash in dynamic string containing Rails.root #1177

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