Fix for supporting nested sections in layouts

[ajaybhargavb]

Issue #2179
@rynowak @pranavkm

[rynowak]

unRenderedSections -> unrenderedSections

[rynowak]

[ajaybhargavb]

Updated with tests.

[pranavkm]

This might not address sections that are redefined but not rendered in the outer definition.

// NestedLayout
@{ Layout = "OuterLayout"; }
@section scripts {
   // stuff
}

// OuterLayout
@{ Layout = "BaseLayout"; }
@section scripts { 
  // Don't render inner scripts
}

// BaseLayout
@RenderSection("scripts");

In this scenario, the scripts section defined by NestedLayout is never rendered but the view would happily succeed. Perhaps we shouldn't be flattening the results in unrenderedSections?