Null string field values parsed as text 'null'

[wabson]

Similar to #50 but for string values, when deserializing JSON data to a model instance, any string property values in the response are converted to the text 'null' on the returned instance.

Type of issue: (check with "[x]")

- [ ] New feature request
- [x] Bug  
- [ ] Support request

Current behavior:

Nulls returned as text 'null'

Expected behavior:

Null values should be returned as the null value

Steps to reproduce the issue:

Request a process instance where the businessKey property or any other is null. Debugging reveals that the property is null in the HTTP response object but when parsed it is converted to text.

Node version (for build issues):

New feature request: