@processContent injecting a template element breaks IE rendering

**I'm submitting a bug report**

* **Library Version:**
1.4.2

* **Browser:**
IE 11

* **Language:**
TypeScript 2.3.3

**Current behavior:**
I recently implemented an approach for hiding the replace-part syntax from our framework's custom controls. @jdanyow pointed me in the right direction [here](https://stackoverflow.com/questions/43306744/using-custom-element-content-as-item-template).
This works fine in chrome/firefox/edge, but now we're facing issues with a customer still using IE11. The templates simply don't render there.

I was thinking this would be the same issue as #460, but unfortunately the fix provided there didn't work in my case.

You can reproduce this by running the attached project in IE and Chrome. The dropdown is populated in IE, but the content doesn't render.
[au_template_issue.zip](https://github.com/aurelia/templating/files/1025720/au_template_issue.zip)