Fix buffer encoded as utf8 string

Any buffer param passed to utils.queryParser.encodeParam that contains a utf8 digit will match the /\d+/ regex, and will be returned as toString(), when it should be returned as toString('hex').

ie:
```javascript
utils.queryParser.encodeParam(new Buffer([0x30, 0x97])) 
```
returns 0ï¿½ when it should return 0x3097