update subtemplate changes bubbling - fixes #109

Fix for issue #109
This issue was actually introduced with the new sub-scopes introduced with the {let} statement. In the sample provided with #109, the {if} statement is creating a sub-scope, and the sub-template property bubbling mechanism was creating a property in the wrong scope (in this example an *index* property was created in the {if} scope and was hiding the *index* property of the {foreach} item)

PS: the sub-template property bubbling mechanism is used to bubble scope changes from a sub-template to its parent template. This is necessary for instance when the sub-template contains *input* elements that can change sub-template scope variables that should be then updated in the parent data model.