Enable passing CloudEvents headers ('CE-*')

[evankanderson]

Also cleans up some go vet warnings.

THIS IS NOT TESTED. Mostly because we don't seem to have any test cases yet for these buses. It seems like it would be useful to have a small test harness that accepts a set of HTTP requests and sends them to the bus then verifies that the correct messages were received by standing up a listening HTTP server and looping back from the bus to the handler.

Works towards #53

Proposed Changes

• Add CE- headers to the whitelist on the two buses that use a whitelist for handling messages. Long term, I'd like to see the buses move towards a blacklist model in terms of header passing, both in light of Make extension attributes siblings to our attributes cloudevents/spec#225 and to enable other development and experimentation without needing to rebuild the controller.
• Added docstrings wherever they were missing, or made methods which were not used outside the package private in the gcppubsub case.

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/assign @scothis
/assign @ericbottard
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[scothis]

@evankanderson I started an effort to centralize the http bit for the buses over at #175. I added support for the common ce-* headers over there, but not via a prefix which we'll need to handle ce extensions.

I can port your change on top of that PR if you'd like to close this one.

[evankanderson]

That sounds fine with me; what's the ETA on that PR?

I did this as part of trying to thread cloud events though the whole system.

What's the rationale behind removing most headers in the bus? It seems like 2 of 3 implementations do this, but it seems hostile to extension (as we're seeing with cloud events).

[scothis]

/hold

[evankanderson]

Closing in favor of #175