Using ActiveSupport#on_load instead of send :require to boot.

This is similar to a [pull request I made][1] a while back on another project that included itself onto `ActionController`. In order for this to work on the `rails-api` gem, and other projects which may not use `ActionController::Base`, the method for including the `ActionController::StrongParameters` module should be changed.

``` ruby
# instead of...
ActionController::Base.send :include, ActionController::StrongParameters
# let's use...
ActiveSupport.on_load(:action_controller) { include ActionController::StrongParameters }
```

This is a redux of #144 without all the nasty whitespace mangling.

[1]:    https://github.com/plataformatec/has_scope/pull/31/files