Fixes compilation errors on RC1 while using Xcode 5 DP5

This avoids the following compiler error, guessing assign is right:

`Property with 'copy' attribute must be of object type.`

It can be reproduced by simply creating an `Empty Application` Xcode project, and then adding a `Podfile` that looks like:

```ruby
platform :ios, '7.0'
pod 'AFIncrementalStore'
```

then doing a `pod install` and opening the `.xcworkspace` will reveal the aforementioned error.

(Using `Xcode5-DP5.app`)