fix ajax uploads with remotipart

Problem with Paperclip/Carrierwave upload & ajax 

Hey folks,

I have some issues with uploading images to related model through ajax in modal window.

And rails receive request like that:
```ruby
Started POST "/admin/brand/new" for 127.0.0.1 at 2014-06-23 15:14:44 -0700 (pid:10944)
RailsDevTweaks: Skipping ActionDispatch::Reloader hooks for this request. (pid:10944)
Processing by RailsAdmin::MainController#new as JS (pid:10944)
Parameters: {"object Object"=>{","=>{"object Object"=>{","=>{"object Object"=>{","=>{"object Object"=>{","=>{"object Object"=>{","=>{"object Object"=>{","=>{"object Object"=>{","=>{"object Object"=>{","=>{"object Object"=>nil}}}}}}}}}}}}}}}}, "model_name"=>"brand"} (pid:10944)
```

I used formData
https://developer.mozilla.org/en-US/docs/Web/Guide/Using_FormData_Objects
And it perfectly works for me.

rails_admin "0.4.9"
jquery "1.10.2"

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