Allow null values when converting to and from wrapper types (in Slim)

Currently all converters are registered for both primitive and wrapper types. These converters assume that neither the argument nor the value are ever null. This causes problems when you want to actually set a property with a wrapper type to null or want to check whether such a property is null.

I know it's bad style to explicitly set and return null, but we need to test legacy code, so there's no way around.

### Example ###

```java
public class Fixutre {
    private Boolean value;
    public Boolean getValue() { return value; }
    public void setValue(Boolean value) { this.value = value; }
}
```

```
| Fixture |
| value | getValue? |
| true | true |
| false | false |
| null | null |
```

The last line in this test fixture will fail.

This pull request aims to fix issue #215.