Fix bugs in USPS tracking

[trishume]

While running batch processing on tracking numbers I discovered a few bugs in active_shipping USPS tracking. This PR fixes two of them:

• Some country names aren't the same as they are in ActiveUtils and have to be mapped separately. I only found 2 out of the 6000 numbers I tested but I have emailed someone at USPS asking if they have a full list.
• One type of event ("Origin Post is Preparing Shipment") doesn't have a time, even though it really should. This currently causes an exception. I handled this by returning an arbitrary date in the past (Jan 1, 2000).

I'm definitely open to discussion on how I handled the lack of date. I imagine lots of code would assume that all events have a time, which is true for everything except this rare event type, which is why I still return a time rather than nil. I chose an arbitrary past date rather than the current date so that the sort by time would do the right thing and put this event in the past.

@kmcphillips @garethson

[garethson]

This is fine. The way we solved a similar problem in shopify is defining a name_alternates array in the countries YAML file (https://github.com/Shopify/shopify/blob/master/db/data/countries.yml#L51), and then we roll that up into another key https://github.com/Shopify/shopify/blob/master/app/models/country_db.rb#L133). It's a bit more of a sustainable long term approach. Ok with this too though.

[trishume]

There's lots of other hashes like this in the same place so I thought I would do it a consistent way, and there should only be a few of them.

[garethson]

⛵

[trishume]

I received a list of the country names USPS uses from someone at USPS and reconciled the code so that it should handle any country they can throw at us.

[kmcphillips]

This is a great approach. Huge. Thanks for digging to find that country list.