Towards #914: Better, but date range still not working. (See xit tests.)

I think this a better, because at least now we're solrizing the broadcast date... but it is still not actually working: The date range is still not being respected on search, but at least now there's a test. I'm not sure when I'll get back to this, so I wanted to document my work up to this point.

Where in Blacklight does that `begin`/`end` syntax comes from? 

(We'll need to resave the documents to solr at some point, but it probably makes sense to see search actually work before investing too much time on this.)