Updating .format_node_value to handle dates

The Problem:

In working with Sufia 0.0.7, when I go to upload a file, I encounter
the following exception:

```
undefined method `gsub' for Mon, 21 Jan 2013:Date
```

What appears to be the case is that the RDF Datastream declares the
:date_uploaded field as type :date. Then, when calling #to_solr,
the original solrizer is expecting to format strings, but ActiveFedora
is passing a Date value for the :date_uploaded's value

One of these systems should handle this, and I believe Solrizer should
properly transform values to their correct format.