Shipwire now returns 401 Unauthorized when bad credentials are provided vs successful response

@csaunders 

Shipwire used to return a 200 with an error in the body when bad credentials were provided, they now return a 401 instead.
If we get an `ActiveMerchant::Shipping::ResponseError`, that would mean we've gotten past the credential verification and credentials are valid. If we get an `ActiveMerchant::ResponseError` with 401, we return false.

I'm not sure what the point is of some of the tests (verifying that the message of the ResponseError equals the message in the body?), so I removed them.