Markdown: Awkward soft break after abbreviation between ( and newline

Test case:
``` bash
echo '(cf.
Foo)' | pandoc -f markdown -t markdown
```
Output: `( cf. Foo)`.

A space has been added after the open parenthesis. More precisely, if the native output format is chosen, we see it's a `SoftBreak`: `[Para [Str "(",SoftBreak,Str "cf.\160Foo)"]]`.

This is a sufficiently rare case that it only occurred once in a 350 page document.
