defined?(ActionDispatch::Static) is always true

[Antronin]

Instead check if we are serving static files, as that will explicitly means ActionDispatch::Static is loaded.

[switzersc]

Also, in Rails 4.1.6, checking for ActionDispatch errors out:

No such middleware to insert before: "ActionDispatch::Static"
/home/rof/cache/bundler/ruby/2.2.0/gems/actionpack-4.1.6/lib/action_dispatch/middleware/stack.rb:125:in `assert_index'
/home/rof/cache/bundler/ruby/2.2.0/gems/actionpack-4.1.6/lib/action_dispatch/middleware/stack.rb:88:in `insert'
/home/rof/cache/bundler/ruby/2.2.0/gems/railties-4.1.6/lib/rails/configuration.rb:68:in `block in merge_into'
/home/rof/cache/bundler/ruby/2.2.0/gems/railties-4.1.6/lib/rails/configuration.rb:67:in `each'
/home/rof/cache/bundler/ruby/2.2.0/gems/railties-4.1.6/lib/rails/configuration.rb:67:in `merge_into'
/home/rof/cache/bundler/ruby/2.2.0/gems/railties-4.1.6/lib/rails/engine.rb:497:in `app'
/home/rof/cache/bundler/ruby/2.2.0/gems/railties-4.1.6/lib/rails/application/finisher.rb:36:in `block in <module:Finisher>'
/home/rof/cache/bundler/ruby/2.2.0/gems/railties-4.1.6/lib/rails/initializable.rb:30:in `instance_exec'
/home/rof/cache/bundler/ruby/2.2.0/gems/railties-4.1.6/lib/rails/initializable.rb:30:in `run'
/home/rof/cache/bundler/ruby/2.2.0/gems/railties-4.1.6/lib/rails/initializable.rb:55:in `block in run_initializers'

But 'serve_static_files' raises a no method error.

[mzsanford]

I solved this a different way in #53, by inspecting the middleware list