Axistech poller not handling "NaN" in JSON response.

[dajtxx]

The Axistech API may put "NaN" for a value in the JSON response, and the poller does not handle this.

Make two changes for this issue:

1. Ignore any key with "NaN" as a value.
2. Add make the response parser testable so it is easier to run failing messages through it and add a unit test whenever a new failing message arrives.