Fix to make sure all values are handled as arrays

@jcoyne this is an alternative way to handle the PR that @cam156 submitted before (https://github.com/projecthydra/active_fedora/pull/486) 

I am updating solr_digital_object to return all values as arrays which I believe was the original intention. The change will affect all types (dates and others)




How about replacing the check with a coercion:

How about coercing to an array here:

```ruby
val = Array(solr_doc.fetch(field_name, default))
```
See below for why.