Revert "Merge pull request #325 from Shopify/remove-variable-incomplete-end"

That pull request broke raw tags with open variable tags. E.g.

{% raw %}
{{
{% endraw %}
{{ 1 }}

This reverts commit fbaabf3b597589fdc249cc989b585c243e91c7ee, reversing
changes made to af24d2c2abc08faa9fb64503980d332c0cbda0b5.
Add regression test for raw tags with open variable tags.