br transform: remove extra space at eol

Hi Dmitri, sorry for submitting these one at a time - am submitting as I notice them.

This fixes a subtle problem with the `br` transform:
```clojure
(md-to-htm-string "foo  \nbar") => "foo  <br> bar" ; Old behaviour
(md-to-htm-string "foo  \nbar") => "foo<br> bar" ; New behaviour
```

The old behaviour can be a problem with center text alignment, etc.

Note that this change required **modifying one unit test**.

Also, it'd be better if we could get `"foo<br>bar"` output (no leading space on the second line), but it wasn't clear to me how to do that and it's also not practically a problem since afaik all browsers will ignore a leading space following the `<br>` tag.

Cheers :-)