Avoid exception for dup on Integer

Prior to Ruby 2.4, if we were to dup an Integer, it would fail with a TypeError.

> 1.dup
TypeError: can't dup Fixnum
	from (irb):1:in `dup'
	from (irb):1

This was confusing because Integer#dup is actually implemented.

> Integer.respond_to? :dup
=> true

However, if we were to freeze an Integer it would fail silently.

> 1.freeze
=> 1

Ruby 2.4 has now included dup-ability for Integer as well.

> 1.dup
=> 1

In Ruby, some object types are immediate variables and therefore cannot be duped/cloned. Yet, there was no graceful way of averting the error thrown by the sanity check when we attempt to dup/clone them.

So now Integer#dup functions exactly the way freeze does -- fail silently and return the object itself. It makes sense because nothing about these objects can be changed in the first place.