Add the Kernel#singleton_class? predicate method

This method is a convenient way to know whether a given object is a
singleton class or not. This feature has been added in Ruby 2.1 (see
https://bugs.ruby-lang.org/issues/7609 for further information).
Added specs for Kernel#singleton_class?
Merge pull request #3102 from robin850/singleton-class-p

Add the Kernel#singleton_class? predicate method