Scala support does not support serializing booleans

[lbustelo]

@vinomaster hit this problem on his demo notebook. If we do something like

channel().set("aVar", false)

or

channel().set("aVar", true)

They are getting sent to the browser as strings. We cannot then use them as expected in a <template is='dom-if'>.

The problem is https://github.com/jupyter-incubator/declarativewidgets/blob/master/kernel-scala/src/main/scala/urth/widgets/util/SerializationSupport.scala#L27 is missing a case of Boolean and converting to JsBoolean.

/cc @wellecks

[lbustelo]

I'm working on this one