Update field stats resource to use bare aggregation calls

Fix #202, #177.

This resolves 2 of the current errors in the `2.4` branch by accounting for the fact that the aggregator calls(in Avocado 2.4+) now return raw values instead of an Aggregator object.

Signed-off-by: Don Naegely <naegelyd@gmail.com>
`stats` will never be `None` since the last condition is a bare `else` above. Rename `stats` to `resp` and you can remove this block.