Add interface filter

[tfausak]

(This is like #181 but into v1.3.0 instead of master.) Fixes #178.

This allows you to define an interface that an input must conform to. In most cases, you don't actually want to do this. Instead you should use a model filter. For instance, if you wanted your input to respond to #each, you could do this:

class EachInteraction < ActiveInteraction::Base
  interface :thing,
    methods: [:each]
end

But what you probably want is an object that implements Enumerable. So you should do this:

class EnumerableInteraction < ActiveInteraction::Base
  model :thing,
    class: Enumerable
end

Why have an interface filter, then? It may be that the classes you're willing to accept don't have anything in common and there's nothing you can do about it. For instance, you could have a serializer input that can take either JSON or YAML. You couldn't do that with a model filter. (Well, not without class: Object.) With an interface filter, you could do it.

class SerializerInteraction < ActiveInteraction::Base
  interface :serializer,
    methods: [:dump, :load]
  def execute
    h = serializer.load('{}')
    h['now'] = Time.now
    serializer.dump(h)
  end
end

require 'json'
SerializerInteraction.run!(serializer: JSON)
# => "{\"now\":\"2014-05-02 21:57:41 +0000\"}"

require 'yaml'
SerializerInteraction.run!(serializer: YAML)
# => "---\nnow: 2014-05-02 21:57:52.900349283 +00:00\n"

[AaronLasseigne]

Don't forget the translation.

[tfausak]

Good catch! I updated the i18n tests (6816075) so things like that can't happen anymore. In fact, we were also missing a translation for the symbol filter (650e8b5). That's kind of embarrassing; it's been around since 0.6.0.

[AaronLasseigne]