`Type.decodeBytes` returns `RubyString` instead of `ByteList`.

It turns out that odd automagical character set promotion can occur
when coercing byte lists into strings.  By returning strings directly,
we side-step this and retain control of our charsets.

The default charset should now always be `ASCII-8BIT`.
Merge pull request #112 from seomoz/phs/bytes-decoded-as-ruby-strings

`Type.decodeBytes` returns `RubyString` instead of `ByteList`.