Correct fix for Slim #1431, #1434, #1435

This is the correct way how to fix the issues #1431, #1434, #1435. Since you want to use a capture helper you have to disable the internal Slim capturing. This is how it is also done in Rails. I don't recommend to monkey patch the Slim::Template how you did in #1435.

Unfortunately your test cases somehow don't cover the capturing very well and the problems described in #1431, #1434, #1435. You should definitely improve that.