Make Rails/RootPathnameMethods autocorrection safe

[r7kamura]

The safety section states the following:

This cop is unsafe for autocorrection because Dir's children, each_child, entries, and glob methods return string element, but these methods of Pathname return Pathname element.

If so, it should be possible to append .map(&:to_s) on autocorrection so that the behavior does not change before and after the autocorrection, and I thought that more users would be helped by providing such a safe autocorrection logic.

In addition, I found that autocorrection to some File singleton methods is also unsafe, which has not been mentioned in the safety section. I thought it would be better to append .to_s to safely handle these cases as well.

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• Wrote good commit messages.
• Commit message starts with [Fix #issue-number] (if the related issue exists).
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• Squashed related commits together.
• Added tests.
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[r7kamura]

I tried this change on a larger project and found another incompatibility between Pathname#join and File.join.

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

I created a separate pull request for this:

• Exclude File.join from Rails/RootPathnameMethods target methods #1001