change updateCart request body to constant

Author: roninopf

swift-sdk/Internal/RequestCreator.swift

@@ -102,7 +102,7 @@ struct RequestCreator {
 
         let itemsToSerialize = items.map { $0.toDictionary() }
 
-        var body: [String: Any] = [JsonKey.Commerce.user: apiUserDict,
+        let body: [String: Any] = [JsonKey.Commerce.user: apiUserDict,
                                    JsonKey.Commerce.items: itemsToSerialize]
 
         return .success(.post(createPostRequest(path: Const.Path.updateCart, body: body)))