Changing selectedAssets to be read-write

[azizdev]

In QBImagePickerController.h this line:
@Property (nonatomic, strong, readonly) NSMutableOrderedSet *selectedAssets;
Should be changed to:
@Property (nonatomic, strong, readwrite) NSMutableOrderedSet *selectedAssets;

So that if you open the image picker again you can assign the selected assets to _selectedAssets
so you can see the old selected items.

Please consider changing it in the next release, Thanks.