Variables (`@something`) in layouts cannot be bound

[heemskerkerik]

Describe the bug

Using a variable expression (@something) does not work in a layout view, while it does in the ‘child’ view.

When providing the value for a variable in the data parameter, it should be possible to bind it in a layout view. Instead, depending on whether ThrowOnUnresolvedBindingExpression is true, it will throw an exception (HandlebarsUndefinedBindingException: something is undefined) or simply bind to an empty string.

Expected behavior:

When I use a variable expression (@something) in a layout view, and I provide that value in the data parameter, I expect it to be bound correctly.

Test to reproduce

[Fact]
public void CanRenderVariablesInCasperLayout()
{
    var fs = new DiskFileSystem();
    var handlebars = Handlebars.Create(new HandlebarsConfiguration()
                                       {
                                           FileSystem = fs
                                       });
    AddHelpers(handlebars);
    var renderView = handlebars.CompileView("ViewEngine/Casper-master/index.hbs");
    var output = renderView(
        new
        {
            posts = new[]
                    {
                        new
                        {
                            title = "My Post Title",
                            image = "/someimage.png",
                            post_class = "somepostclass"
                        }
                    }
        },
        new
        {
            blog = new
                   {
                       url = "http://someblog.com",
                       title = "This is the blog title"
                   }
        }
    );

    var cq = CsQuery.CQ.CreateDocument(output);
    Assert.Equal("This is the blog title", cq["section.copyright a"].Text());
}

Other related info

Editing the template so the expression is no longer a variable and providing the data in the context parameter does work.

[oformaniuk]

Related to #97 as might become obsolete if view engine redesign comes first.