Bugfix: prevent multiple encoding in nested views/templates

Hi,
the `RENDERING` flag introduced in #656 didn't work with multiple nested levels. For example, in such a configuration:
```
//main.html
<include href="sub1.html"/>
<include href="sub2.html"/>
```
the `RENDERING` flag would be erased after rendering sub1.html and therefore all variables would be double encoded in sub2.html

This pull request intends to fix that.