The splice family of Linux system calls can transfer data between file
descriptors without the need to copy data into userspace. Instead of a
userspace buffer, they rely on an ordinary Unix pipe as a kernel-level
buffer.

* http://bogomips.org/ruby_io_splice/
* ruby.io.splice@librelist.com />* git://bogomips.org/ruby_io_splice.git

Changes:

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.