Preserve nillable annotation attribute

Currently this:
```
@XmlElement(nillable = true)
protected ArrayOfstring field;
```

Becomes this:
```
@XmlElementWrapper(name = "field")
@XmlElement(name = "string", namespace = "http://schemas.microsoft.com/2003/10/Serialization/Arrays")
protected List<String> field;
```

Which changes serialization behavior for null values. Attached commit fixes the problem.