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)




If coerced to array (per above), these two lines collapse into `val`