added improved po_box handling

[ellery]

No description provided.

[ellery]

Before:

 StreetAddress::US.parse("PO Box 163, Alexandria, VA, 22303")
 => #<StreetAddress::US::Address:0x007ffad0b2a8e0 @unit_prefix="Box", @unit="163", @street="Alexandria", @redundant_street_type=true, @street_type=nil> 

after:

 StreetAddress::US.parse("PO Box 163 Alexandria VA 22303")
 => #<StreetAddress::US::Address:0x007f89d99ec1f8 @unit_prefix="Box", @unit="163", @city="Alexandria", @state="VA", @street="22303", @redundant_street_type=true, @street_type=nil> 

[seabre]

I mentioned this in slack, but If you're expecting that unit key value to be PO Box, the test should be testing for it to be PO Box it should fail until it's fixed. Context: https://crowdfiber.slack.com/archives/C4RMP4CJE/p1520285117000599