I was getting `ActionView::Template::Error: undefined method 'number?' for #<ActiveAdmin::Mongoid::Document::ColumnWrapper:0x0000000bfa4ca8>`
for 
```ruby
= f.input :email, required: true, autofocus: true, hint: false, label: false, input_html: { placeholder: "Email" }
```
not sure what the reason was, so this is rather a quick fix.