Unexpected whitespaces caused by <br>

- html2text version: `2018.1.9`
- Python version `3.6.4`

Using the following code I get unexpected whitespaces in front of each `
` which got replaced by `\n`:
```
>>> import html2text
>>> h = html2text.HTML2Text()
>>> h.single_line_break = True
>>> h.handle("foo


bar")
'foo \n \n \nbar\n\n'
```

Additionally the `single_line_break` setting doesn't seem to have any effect at all.