Header field names should all be lower-cased in Version model

[Mr0grog]

Header field names are not case-sensitive in HTTP, so to make analysis of our data easier, we should just ensure we store them as lower-case in the Version model. Today, we’ve got some that are mixed, which is not great.

We should probably do this as a normalize or before_save hook. Then we’ll need to go through and update all the existing versions imported in the past few weeks (the older IA import script lower-cases everything as a consequence of using the Wayback package’s memento.headers dict, but our newer WARC import script did not do this until yesterday).