Test whether a Ruby class is a subclass of another class

I would like to test whether a class inherits from another class, but there doesn't seem to exist a method for that.

class A
end

class B < A
end

B.is_a? A 
=> false

B.superclass == A
=> true

(N.B. the is_a? test above does not make any sense, as the first comment correctly points out B.new.is_a?(A) would make sense, but is not generally applicable, as not every class has a #initialize method that accepts 0 arguments)

A trivial implementation of what I want would be:

class Class
  def is_subclass_of?(clazz)
    return true if superclass == clazz
    return false if self == Object
    superclass.is_subclass_of?(clazz)
  end
end

but I would expect this to exist already.