1026 allow solr lookup by slug field

1. Don't know when to use symbol keys or string keys. Unfortunately it's not up to us, since the type of `@document` as returned by `get_solr_response_for_doc_id_or_slug` is merely the same object returned from either `get_solr_response_for_doc_id` or `get_solr_response_for_field_values`, both of which are located in `Blacklight::SolrHelper`. The type returned is a `SolrDocument` which behaves like a `Hash`, and it may very well behave like a `HashWithIndifferentAccess`. Not sure.

1. Yes the `rescue` is there for when `document==nil`, and in reviewing this code.. .it always will be `nil`, since it's `@document` that we want to look at. I'll change per your suggestion.