fix #16482 Regression: _WidgetBase#placeAt() can't be used on a

Added DOH tests for `dom-construct.place` in https://github.com/dojo/dojo/pull/68

DocumentFragment support in `WidgetBase.placeAt`

* added tests for placement using document fragment
* updated WidgetBase.placeAt to allow a document fragment

[fixes 16482](https://bugs.dojotoolkit.org/ticket/16482)