fix issue with symbolic object names using SimpleForm

[jfeaver]

I was using an asset uploader on an Admin form using simple_form (simple_form_for [:admin, user]) and getting this error where no error exists for the same input on a non-admin form:

ActionView::Template::Error (undefined method '+' for :user:Symbol)

effective_assets (1.4.9) app/views/asset_box_input/_attachment_as_thumbnail.html.haml:19:in `___sers_nathan__rvm_gems_ruby________lam_allery_gems_effective_assets_______app_views_asset_box_input__attachment_as_thumbnail_html_haml__308308914485762670_70167524492620'
actionview (4.2.1) lib/action_view/template.rb:145:in `block in render'
activesupport (4.2.1) lib/active_support/notifications.rb:166:in `instrument'

[matt-riemer]

Great fix, thanks!!