Online::Invoice: Test that id and sales_term_id are nil if not present
Fix test for Windows::Invoice so it's actually using the right class
Windows::Invoice: Test that id and sales_term_id are nil if not present
Only turn on SimpleCov with COVERAGE env var set
Prevent 'undefined namespace prefix' error

This will happen with the Windows services if the response does not
contain any tags with a namespace (say, if the response body is
something random).
Replace logging with hooks

Instead of logging the request/response, add an 'on_request' hook to
all service classes. It is then up to client code to do whatever it
wants to with the request and response objects.