This now actually extracts the path from the IO if one is passed in.

It's already required in zip.rb.

Things are now more carefully set up, and if a buffer is passed in which
represents a file that already exists then this is taken into account. All
initialization is now done in File.new, rather than being split between there
and File.open_buffer.

This has also needed a bit of a re-write of Zip::File.initialize. I've tried to
bring some logic to it as a result, and have added comments to explain what is
now happening.

StringIO objects created within File.open_buffer were not being switched into
binmode, but those passed in were. Fix this inconsistency and add a test.