Pass blocks sent to doubles along to the stubbed block via block arg

Currently, any blocks that are passed to methods on the double get dropped in `TestDouble#method_missing`. I would like to pass along blocks so I can test cases where a collaborator provides a method that takes a block. For example,

```ruby
class Collab
  def wrapper(&block)
    wrap
    block.call
    unwrap
  end
end

class Sut
  def initialize(wrapper)
    @wrapper = wrapper
  end

  def behavior
    @wrapper.wrap { more_behavior }
  end

  def more_behavior
    change_some_state
  end
end

class SutTest
  def test_behavior
    wrapper = gimme
    give(wrapper).wrap {|blk| blk.call }

    sut = Sut.new(wrapper)
    sut.behavior
    assert_equal new_state, sut.state
  end
end
```

My rspec is a little rusty (I've been using gimme a lot :) so hopefully the included test is acceptable. Let me know your thoughts or if I could tweak anything to get this into a state you'd be comfortable merging. Thanks!