Fix UploadedFile encoding error from to_json

When I'm trying to use an example where I serialize a PDF I get the following error:

```
/Users/Blake/.rbenv/versions/2.1.4/lib/ruby/gems/2.1.0/gems/activesupport-4.1.6/lib/active_support/core_ext/object/json.rb:34:in `encode': "\xE2" from ASCII-8BIT to UTF-8 (Encoding::UndefinedConversionError)`
```

Oddly enough this only happens on the first time you try to call `to_json`.

I know it's not ideal, but this is definitely a temporary and quick fix until I or someone else has time to implement a better fix.