Remaining fixes to pass all of MRI's test_stringio.rb.
Twiddling some specs that fail now (as they do in MRI).
Additional tweaks and fixes for StringIO

* Fix bug related to porting (http://bugs.ruby-lang.org/issues/8728)
* Actually throw arg length error, even though invoker logic
  will do it (consistency with MRI port)
* Cleanups
Add StringIO#codepoints and untag passing specs.