Fix default from date to upcoming stats filter

This is a tweaked version of https://github.com/alphagov/whitehall/pull/1635, which was reverted in https://github.com/alphagov/whitehall/pull/1641.

The search API expects `from` to be a `Date` (as per https://github.com/alphagov/whitehall/pull/1641#issuecomment-51045038), this is fixed in https://github.com/alphagov/whitehall/commit/ced2eb28b45851b9d48f1976781255eed62acc54.

This has been tested against live search index on my dev env, and worked, and I was able to replicate the old version failing against that, which it wasn't against the test DB.

I'm not familiar with the nuance of the original change, but this should fix the bug in it.