Remove trailing whitespace in record/base_test
Add necessary methods to mocks

Some of the unit tests in record/base_test use mocks and stub
class definitions instead of real model/record classes. This commit
adds class-name constants instead of anonymous classes where necessary
(because of const_get calls in the implementation) and makes parent
mocks quack like Xeroizer::Record::BaseModel instances.
Remove trailing whitespace
Use correct status values for bank transactions

Xero API docs give allowed status codes as AUTHORISED, DELETED. (Also
nil, since status is an optional field.)

Also, validate bank transaction status on both new and existing records.