#91 Regression spec for empty field with parameter group

Here's the failing spec, if it makes sense then I'm happy to look at a fix.

I also created a branch pinned against grape 0.6.0, prior to the regression, see diff [here](https://github.com/dukedave/grape-swagger/compare/dukedave:master...grape-0.6.0) (caveat: I also had to remove the `type` option which wasn't supported back then).
If you check it out you should see the new spec passes, unlike on master.