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Detect BasicObject more accurately
Check for ::Erubis::Eruby in Erubis template

If the Erubis module had been defined but not the Eruby class (which
can happen if you require erubis/tiny) the Erubis template fails with an
"uninitialized constant" error.

Check for the specific Eruby class rather than just the containing
module in `engine_initialised?`
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