Revert "Move frozen checks to getSingletonClass() - this allows for MRI-compliant behavior"

This reverts commit 7ff0a1f47358e834d52ea1473b235ec83c8d19c6.
Fix freeze enforcement and exceptions for immediate values. Passes test
suite and rubyspec suite now.
Merge pull request #2178 from cheald/fix_frozen

Fix frozen checks and exceptions