eliminate extra blank line in verbatim blocks

I get extra lines in the markdown output when I include bllank lines in my verbatim blocks.  Example (using $ for EOL):

```
    line one$
    line two$
    $
    line three$
```

becomes

```
    line one$
    line two$
    $
$
    line three$
```

I don't see this extra line with other formatters.  Caveats:

1. the bug (if it is one) may actually be in Pod::Parser
2. The patch provided uses a private attribute _PREVIOUS and may not be kosher

but at least the test case should be useful if there is a better solution.