Convert Authorize.net doDirectPayment to use guzzle & add test

Convert Authorize.net doDirectPayment to use guzzle & add test

This brings the main Authorize.net payment function under testing, putting us in a better position to bring it
into line with more recommended coding practices.

Note the trick to writing tests for Guzzle is to first add the lines

```
    $this->setUpClientWithHistoryContainer();
    $this->processor->setGuzzleClient($this->getGuzzleClient());
... do the stuff

   $responses = $this->getResponseBodies();
   $requests = $this->getRequestBodies();
```

That allows you to capture the outgoing & incoming responses that form the expectations.
Obviously some credential wrangling is needed to get an OK response but it's also possible
with some processors (e.g ones that are basically done for) to just worry about ensuring
the outgoing is captured