Fix ServiceCheck none parameters for calls from dogshell, etc on python3.

[timvisher]

#206 (comment) introduced a basic python3 error where we attempted to iterate over a dictionary we were modifying, yielding an exception.

We didn't have any unit tests around this so I added one. This fixes both the actual call that I was making and provides a regression test.

I'm happy to clean up the commits once I get basic validation that this is the right direction.

If it's more important than I realize to actually filter out the None params, we'll need to change the way that we do that (iterating over keys and building up a new dictionary with the appropriate key/value pairs.

[yannmh]

Thanks @timvisher 🙇 .

Cleaning out the None parameters is not necessary anymore.