A tweak to make flags with default values act as switches when passed without args

When a script is configured with:

```
opts = Trollop::options do
  opts :argwithdefault, "An Argument w/ a default", :default => "default value"
end
```

Then trying:
```
command --arg-with-default
```

Seems like we should get the default value rather than an error.

via [gitorius](https://gitorious.org/trollop/mainline/merge_requests/11)