A number of minor updates to get tests passing again.

* Correct previous encoding fix to work on older rubies.
* Force encoding of input IO to be ASCII_8BIT. Fixes broken tests on
  ruby-1.9 and jruby caused by StringIO returning US_ASCII instead.
* Specify file encoding so string comparison tests pass on ruby-2.0.
* Relax stat check in test_filesystem for jruby.
* Move new utf8 string write test to be in OleUnicodeTest in
  test_filesystem, as it uses the filesystem api.
Update ChangeLog and bump version.