Define $xml to pass xml object

In my project, I needed to pass a xml object in my envelop SOAP. 

So, I add '$xml' variable as '$value' without xmlParser function.  

Example : 
-- JSON in
```javascript
input.dom = {
     $xml: '<test type="type"><ctx></ctx></test>'
};
```
-- XML out
```xml
<tns:dom>
    <test type="type">
          <ctx></ctx>
    </test>
</tns:dom>
```