Fix stack propagation bug in sections

[Arjav-Patel]

Consider the following mustache template:

Template = <<"
{{# list }}
{{# util.titlecase }}
{{# util.to_lower }}
{{ . }}
{{/ util.to_lower }}
{{/ util.titlecase }}
{{/ list }}
">>.

With the following data:

Data = #{
     "list" => ["ELEM 1", "ELEM 2", "ELEM 3"],
     "util" => #{
            "to_lower" => fun(Text, Render) -> string:to_lower(binary_to_list(Render(Text))) end,
             "titlecase" => fun(Text, Render) -> string:titlecase(binary_to_list(Render(Text))) end
      }
}.

If we run:

bbmustache:render(Template, Data).

We expect the following output:

Elem 1
Elem 2
Elem 3

But instead, we see the following:

This is because the stack context is not properly propagated when bbmustache iterates through lists.

Here is a JS fiddle that shows this working properly in mustache.js.