Change ruby bug to actual spec on Module#define_method.

Ruby bug 2117 has been fixed in MRI 1.8.7 branch, so this should be
updated too.
Add spec on Module#define_method with singleton class in 1.9 mode.
Add specs on Module#define_method with UnboundMethod body.
Fix Module#define_method with UnboundMethod body.
Fix Module#define_method with singleton class in 1.8 mode.
Merge remote-tracking branch 'razielgn/define_method_unbound'
Cleaned up method names for bindable methods.