Convert with default attrs

I was having problems creating a rpm package from a gem using FPM::Package#convert because it did not know the default values of the attributes required by rpm, _e.g._  compression, user, group.

I think I found a bug, and I think this is the solution. (Plus a spec to show what I am talking about.)
Please see also this gist https://gist.github.com/jkanywhere/28c623fb33e00cba3365