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.

The test is written the way it is to avoid embedding multibyte characters in the source file.