check against undefined result in #requestAll

received error *TypeError: Cannot read property 'next_page' of undefined at `node-zendesk/lib/client/client.js:202:28`*

seems that `result` sometimes come back undefined and dies when anything is called on it. 
line 118 checks for null, below is the result: Looks like JSON with an embedded buffer? i have never seen that...  

looks like this on all requests

```bash

{ '0': 
   { statusCode: 200,
     result: <Buffer 7b 22 75 73 65 72 73 22 3a 5b 7b 22 69 64 22 3a 33 36 36 36 36 32 31 30 32 2c 22 75 72 6c 22 3a 22 68 74 74 70 73 3a 2f 2f 64 61 74 61 68 65 72 6f 2e 7a ...> } }


```