Markdown filter: put back in trailing spaces to handle implicit <br>
Fixed the markdown unit test
newline ambiguity resolved in reading raw_haml
Merge pull request #138 from boscoh/markdown_br_fix

Fix Markdown br bug
Thanks to @boscoh for the following:
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.