Tidying up hash key access of AF::Model

Instead of using:
```
attrs.has_key?(key) ? attrs[key] : DEFAULT
```

Use:
```
attrs.fetch(key, DEFAULT)
```

And removing the ever confounding unless/else/end is a good thing
Merge pull request #151 from projecthydra/cleanup-active-fedora-model

Tidying up hash key access of AF::Model