be_extension_of matcher.rb

Spec::Matchers.define :be_extension_of do |expected|
  match do |obj|
    (class << obj; self; end).ancestors.include?(expected)
  end
end

obj = Object.new
obj.should be_extension_of(Kernel) # => true
obj.should be_extension_of(Spec)   # Spec::Expectations::ExpectationNotMetError
obj.extend(Spec)
obj.should be_extension_of(Spec)   # => true