- use ruby_run_node(node) on ruby-2.x.
- NSString#fileSystemRepresentation returns regular string.
on ruby-2.x, a return value type (char *) from Objective-C becomes Ruby
String with dummy encoding "RUBYCOCOA_UNKNOWN".
NSString#fileSystemRepresentation is used in most of rb_main.rb.
If the method returns regular string, we need not to modify rb_main.rb
of out RubyCocoa apps.
