Fast specs

[haines]

a039671 adds a Draper::ViewContext.test_strategy method. The default setting is test_strategy :full, which retains the current behaviour (create an ApplicationController and use its view context in the tests).

Using test_strategy :fast eliminates this dependency and instead uses a subclass of ActionView::Base, allowing decorators to be tested in isolation as discussed in #424.

If some of the application's helpers are required, they can be selectively included by passing a block:

Draper::ViewContext.test_strategy :fast do
  include ApplicationHelper
end

I also fixed a minor bug (3d07cb3) which potentially introduces order dependencies into tests. The view context was being set in controller and mailer specs but not cleared afterwards; decorator specs executed subsequently were using that view context rather than building one.

[steveklabnik]

Awesome. :)