How do I check block given?

mock_block_spec.rb

require 'rspec'

class Foo
  def bar &block
    block.call(:called) if block
    :done
  end
end

# Foo.new.bar{ |m| p m }
# # >> :called

describe Foo do
  describe '#bar' do
    context 'given block' do
      it 'should call block' do
        block = double()
        block.should_receive(:call)
        Foo.new.bar &block
      end
    end
  end
end

# /Users/niku/junk/2011/03% rspec mock_block_spec.rb
# F
#
# Failures:
#
#   1) Foo#bar given block should call block
#      Failure/Error: Foo.new.bar &block
#        Double received unexpected message :to_proc with (no args)
#      # ./mock_block_spec.rb:19:in `block (4 levels) in <top (required)>'
#
# Finished in 0.00087 seconds
# 1 example, 1 failure