io_splice 4.1.0 - copy_stream enhancement for 1.9

IO::Splice.copy_stream coerces based on the availability of
the #to_path method in source and destination arguments.

This is to be compatible with IO.copy_stream and also
Rack::File since this may be used in web servers.

This is NOT visible with Ruby 1.8 since File.open
doesn't coerce with #to_path on the given object.