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 />\n bar" ; Old behaviour
(md-to-htm-string "foo  \nbar") => "foo<br />\n bar"   ; New behaviour
```

The old behaviour can be a problem with center text alignment, etc.

Note that this change required **modifying one unit test**.

There's two additional improvements we could make, but that are less important:
1. `"foo<br>\nbar"` output (no leading space on the 2nd line) would be preferable though isn't practically a problem since afaik all browsers will ignore a leading space following the `<br>` tag. Not sure if there'd be any easy way of doing this?
2. The `br` transform should only actually apply when there's 2 spaces _preceding a newline_. Currently (as in the unit test) it applies even on a single line. Also not a big issue in practice, but it would be nice if we could fix that. Any ideas?

Cheers :-)