Implement Module#singleton_class? predicate method

Hello,

This pull request simply adds the `Module#singleton_class?` method to JRuby. This method has been introduced in Ruby 2.1 and is a convenient way to check whether a class is a singleton one or not (see https://bugs.ruby-lang.org/issues/7609 for further information).

A RubySpec has been added for that in rubyspec/rubyspec#279.

Have a nice day.