Updated test doubles (markdown)

+Let's make a few changes to our design of `Converter`. First off, let's allow that it's easier to

+provide configuration parameters via a hash. We'll also provide sensible defaults -- a specific converter

+that does nothing and `nil` for the logger. We won't trigger a logger if no logger is set.

+

+```ruby

+class Converter

+ class PassThroughConverter

+ def convert(value)

+ value

+ end

+ end

+

+ attr_reader :converter, :logger

+

+ def initialize(args = {})

+ @converter = args[:converter] || PassThroughConverter.new

+ @logger = args[:logger]

+ end

+ def convert(value)

+ converted_value = converter.convert(value)

+ log(value, converted_value)

+ converted_value

+ end

+

+ private

+

+ def log(value, converted_value)

+ logger.log(value, converted_value) if logger

+ end

+end

+```

+

+Now, what do we want to verify? We do *not* want to verify that the `log` method on `Converter` gets

+called with `value` and `converted_value` -- what we want to know is whether the collaborator is

+sent the right message. In this case, we want to know if a logger instance would be sent

+the message `log` with the right values. At this point, we don't even have a logger class. We just

+know that it is going to expose a method `log` and expect that the method call will pass the

+value and its conversion.

+

+To make this happen, we want to create a `Mock`.

+

+

+