Handle SOAP Response element value and attributes so that the attribu…
…tes of an element don't get overwritten if also a value is given.

In case a value and attributes are given, the JSON response will return an `Object` with `$value` and `attributes` (in respect to the  set `attributesKey` so that it could also be `$attributes` or something the like) instead of a `String`.
Add support for the `elementFormDefault` schema attribute.