Exclude File.join from Rails/RootPathnameMethods target methods

[r7kamura]

File.join and Pathname#join work differently:

$ irb -r pathname
irb(main):001:0> File.join('/foo', '/bar')
=> "/foo/bar"
irb(main):002:0> Pathname.new('/foo').join('/bar').to_s
=> "/bar"

This can cause false positives for code that intentionally uses the difference. Since this cop claims to be safe, I think it would be better to exclude File.join from its inspection target.

I noticed this difference while working on the following Pull Request:

• Make Rails/RootPathnameMethods autocorrection safe #993

---

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.

[koic]

The detection should occur unless the 2nd argument+ of File.join starts with a /.

Pathname.new('/foo').join('/bar').to_s # unexpected "/bar"
Pathname.new('/foo').join('bar').to_s  # expected "/foo/bar"

See for details: https://bugs.ruby-lang.org/issues/19155