Swift Compatibility: NSString * to NSAttributedStringKey *

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As of swift and objc NSAttributedStringKey differences, there it is typedef of NSString for Obj-C and struct not subclassed from String in swift.

Just declaring explicitly required type will allow to use it in swift wihtout issues.

@property (nonatomic, strong, nullable) NSDictionary<NSString *, id> *defaultAttributes;
@property (nonatomic, strong, nullable) NSDictionary<NSAttributedStringKey *, id> *defaultAttributes;