Using ActiveSupport#on_load instead of send :include 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 }
```

Also, it appears my editor has trimmed up some trailing whitespace in the rest of the file; let me know if that's a problem with this PR and I can change it.

[1]:    https://github.com/plataformatec/has_scope/pull/31/files