Closing Section fixes and changes

Fix keypath expression sections:

    {{#foo[bar]}}{{.}}{{/}}

Allow empty ref closing sections:

    {{#items}}{{.}}{{/}} //this now works

But, look for mismatches on reference closing sections. This:

    {{#foo}}{{.}}{{/bar}}

now raises:

    Could not parse template: Illegal closing section {{/bar}}. Expected {{/foo}}.