prevent heading line style affecting text afterwards

if headings are followed by text elements that are not wrapped inside tags, lineAttribute never gets clearer.

to reproduce, create a test pad and: 

```
curl -d apikey=`cat APIKEY.txt` -d 'padID=test' -d 'html=<div><h1>foo</h1>^M<br>^Mfnordfofowersdf</div>' 'http://localhost:28002/api/1/setHTML'
```