Finish the span when subscription is canceled

[NHebrard]

Hello!

With this PR, spans bound to canceled subscriptions will be also finished. Cf this issue: #133

As span finishement is a key behavior, I added a set of tests asserting this action for all the final signals: complete, error, and canceled.

Thanks @geoand.

[geoand]

Thanks for the PR!

Can you please fix the licensing headers of the files you changed? You just need to adjust the "to" year to 2020

[NHebrard]

Should be fine.