Fix FilePart length calculation for Ruby 1.9 when filename contains multibyte
characters.

Applicable RFCs (http://en.wikipedia.org/wiki/MIME#Encoded-Word,
http://stackoverflow.com/questions/1361604) indicate only ASCII8BIT is allowed
in headers and suggest encoding options, but Chrome and Firefox put multibyte
characters in the header and the Rails stack handles it correctly, so ... just
fix the length.
Fix multibyte filename test for Ruby 1.8 and Jruby
Merge pull request #21 from hexfet/master

Fix FilePart length calculation for Ruby 1.9 when filename contains multibyte characters