Fix for https://github.com/mzabriskie/axios/issues/248
Fix for https://github.com/mzabriskie/axios/issues/248

undefined is not valid JSON, make the value null instead.

JSON.parse(undefined) will throw an exception while JSON.parse(null) is acceptable.

http://stackoverflow.com/a/14946821/5012948
axios/axios,axios/axios,akshatnitd/axios,nickuraltsev/axios,renzhaozhao/axios,mzabriskie/axios,mzabriskie/axios,renzhaozhao/axios,axios/axios,akshatnitd/axios,akshatnitd/axios,renzhaozhao/axios,nickuraltsev/axios,nickuraltsev/axios,mzabriskie/axios