Two minor fixes for the MWSClient

Now MWSClient is using the locale constructor parameter but later overrides the values with what is specified in options and or default values. First commit fixes this.

The second is fixing inconsistent behaviour of .getNext for NextToken calls. There are two substantial problems. First one is when getNext is invoking callback directly with response object, while for example listorders will pass (orders, response) . I think it makes more sense to be able to process the data with the same callback. Second once .getNext is called the response it passes to the callback is missing .nextToken  because  the invoke in .getNext is missing the nextTokenCall parameter.