Fix Markdown br bug

In the `:markdown` filter, the markdown for implicit `<br>` tag is broken.

The reason is that markdown recognises a line with  `<br>` by two trailing spaces.

However, trailing spaces are stripped when `PlaintextNode.haml` is read.

To fix: when the text is reconstructed to send to the Markdown filter, instead of using the `PlaintextNode.haml` field, `PlaintextNode.raw_haml.lstrip()` is used, which keeps the trailing spaces.