Fixed backward incompatibility for engines.
Fixed backward incompatibility for engines.

- Many engines rely on being able to join directories to the Rails root:

    Rails.root.join('somedir')

- This was now impossible because Rails.root returned a String:

    NoMethodError: undefined method `join' for "/code/myrailsapp":String

- This was broken in 4001835db00ce44cb75bca33ec02cd76b8ccc790

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