(PUP-3388) Fixed Zpool Provider for the mirror attribute

Ticket associated with this pull request: https://tickets.puppetlabs.com/browse/PUP-3388

Without this patch applied, multiple drives with mirrors could not be added.
This is a problem because the resoruce specifies that it could handle these conditions.

With the 1 line of code that I removed, and the 2 lines of codes that I added
I was finally able to bring the resource upto the standards of what the
resource was intended to do.
Merge branch 'ticket/master/PUP-3388-zpool-mirrors' of https://github.com/Incognito1992/puppet into ticket/master/PUP-3388-zpool-mirrors

Conflicts:
	spec/unit/provider/zpool/zpool_spec.rb