Fix an issue preventing interoperability with Swift

Optional values may be set to `null` in JSON. When the data is parsed
this turns into `NSNull` which isn’t equal to nil. Setting the
attribute to `NSNull` leads to a type mismatch so it needs to be
included in the comparison so that scenario can be prevented.

Author: christianklotz

Classes/JSONAPIResourceParser.m

@@ -205,7 +205,7 @@ + (void)set:(NSObject <JSONAPIResource> *)resource withDictionary:dictionary {
 
         } else {
             id value = [attributes objectForKey:[property jsonName]];;
-            if (nil == value) {
+            if ((id)[NSNull null] == value) {
                 value = [dictionary objectForKey:[property jsonName]];
             }