Support multiple properties with be_zfs matcher and fix edge case

I've fixed the edge case mentioned in #40 .

And also I've added a feature that be_zfs.with takes multple properties like this.

```ruby
it do
  should be_zfs.with(
    'mountpoint'  => '/rpool',
    'compression' => 'off'
  )
end
```

@ftnk , I appreciate if you check them.