Convert GString to String when bridging to jelly.

[Vlatombe]

When using a groovy view and passing a groovy expression as
"${e1}${e2}" to a taglib, the resulting value is not coerced to a
String, which can lead to incorrect results when calling functions from
within jelly.

For example, when declaring a widget in Jenkins, using the following index.groovy as view

def l = namespace(lib.LayoutTagLib);

l.pane(width:2, title:"Title", id:"${my.urlName}", collapsedText:"CollapsedText") {
  text('Foo')
}

In the resulting map that is passed to JellyBuilder, id has type GStringImpl, unlike title or collapsedText which get coerced to String.

Then in https://github.com/jenkinsci/jenkins/blob/62546443947fafecc94a5e3b47bb9129bed8e148/core/src/main/resources/lib/layout/pane.jelly#L51, the function call doesn't happen because attr.id is a GString, not a String.

@jglick