Make the various occurrences of ftype consistent, and aligned to ::File.ftype

[hainesr]

This discussion started in #530 with @fbacall.

We should be consistent with ::File.ftype on what we return for entries in a Zip archive.

Notes:

• ::File.ftype returns a string: 'file', 'directory' or 'link' (others, e.g. 'socket' aren't relevant for Zip files)
• Zip::Entry.ftype returns a symbol: :file, :directory or :symlink
• Zip::FileSystem::File.ftype returns a string, but returns 'file' if the entry is a symlink
• Zip::FileSystem::File::Stat.ftype returns a string, and an error (Unknown file type) if the entry is a symlink