Record identified calls when one is missing

Here's the scenario:

1. Today:
1. Get access token (recorded).
1. Get data with access token (recorded).
1. Tomorrow:
1. Get access token (replayed, same user).
1. Get data with access token **fails because of `401`**.

The problem here is that we replay a fixture for that expired access token, but the new data calls need a _valid_ token.

A couple ways to remedy this:

- The easiest way is that the `normalizer` should not `delete` any properties (e.g. `sig`, or possibly a date) that would replay old tokens for new requests.
- The harder way is to re-record all previously replayed requests that share that same identity. (However, what will this mean for previously-recorded responses that depend on those headers?)