Modify task extensions to support backgroundTask

[amongonz]

I noticed the for-loop extension to the task CE doesn't work with backgroundTask because the code extends TaskBuilder instead of the more general TaskBuilderBase, from which both TaskBuilder and BackgroundTaskBuilder inherit.

This PR does the small change to support backgroundTask as well.

[abelbraaksma]

Oops, your PR went unnoticed for some time, sorry about that!

Thanks @amongonz for finding this bug and tackling it. I will need some accompanying tests to ensure that if anyone changes this again, it will break. I think a single test that didn't compile before with BackgroundTask and now does work with it, would suffice to tackle this.

Don't worry if you don't have time to add a test, I can do that as well ;).