jsonToXml don't serialize Date value as ISO8601 format.

Nobody has claimed this yet.

I found strong-soap's XMLHandler use `toString`, not `toISOString` for serializing `xsd:dateTime` value in request arguments.
I think string format for `xsd:dateTime` is ISO8601. (format: `[-]CCYY-MM-DDThh:mm:ss[Z|(+|-)hh:mm]`)
http://books.xmlschemata.org/relaxng/ch19-77049.html

Here is my sample code:
```javascript
var handler = new soap.XMLHandler();
var node = handler.jsonToXml(null, null, soap.XMLHandler.createSOAPEnvelopeDescriptor("soap"), {"date": new Date()});
var xml = node.end({pretty: true});
console.log(xml);
```

Expect:
```
2017-04-18T15:24:26.871Z
```

Actual:
```
Wed Apr 19 2017 00:24:26 GMT+0900 (JST)
```

Start in XMLHandler.jsonToXml and reproduce the provided sample with a Date value. Compare the generated XML with the expected ISO8601 xsd:dateTime format; the issue is done when Date values serialize with that format instead of the shown Date.toString output.

Written by the indexing model from the issue text.