fixed: set nullPointer

[danielsss]

• redis-mock is going to crash if calling set('key', null)

  • (null || undefined).toString() got a fatal error
  • String(null || undefined) is a good way to fix it

• Error

  should set `null` as value:
   TypeError: Cannot read property 'toString' of null
    at parseArguments (/Users/danielssun/Projects/redis-mock/lib/redis-mock.js:39:29)
    at RedisClient.set.RedisClient.SET (/Users/danielssun/Projects/redis-mock/lib/redis-mock.js:353:16)
    at context.<anonymous> (/Users/danielssun/Projects/redis-mock/test/redis-mock.strings.test.js:47:7)
    at Test.Runnable.run (/Users/danielssun/Projects/redis-mock/node_modules/mocha/lib/runnable.js:233:15)
    at Runner.runTest (/Users/danielssun/Projects/redis-mock/node_modules/mocha/lib/runner.js:387:10)
    at /Users/danielssun/Projects/redis-mock/node_modules/mocha/lib/runner.js:470:12
    at next (/Users/danielssun/Projects/redis-mock/node_modules/mocha/lib/runner.js:312:14)
    at /Users/danielssun/Projects/redis-mock/node_modules/mocha/lib/runner.js:322:7
    at next (/Users/danielssun/Projects/redis-mock/node_modules/mocha/lib/runner.js:257:23)
    at Immediate._onImmediate (/Users/danielssun/Projects/redis-mock/node_modules/mocha/lib/runner.js:289:5)
    at runCallback (timers.js:810:20)
    at tryOnImmediate (timers.js:768:5)
    at processImmediate [as _immediateCallback] (timers.js:745:5)