feat: accept aes128gcm content encoding

[jrconlin]

closes #930

NOTE: pjenvey: If this conflicts with your refactor, this has lower priority.

[codecov-io]

Codecov Report

Merging #932 into master will not change coverage.
The diff coverage is 100%.

@@          Coverage Diff          @@
##           master   #932   +/-   ##
=====================================
  Coverage     100%   100%           
=====================================
  Files          57     57           
  Lines        9419   9464   +45     
=====================================
+ Hits         9419   9464   +45

Impacted Files	Coverage Δ
autopush/router/gcm.py	100% <100%> (ø)	⬆️
autopush/tests/test_integration.py	100% <100%> (ø)	⬆️
autopush/router/fcm.py	100% <100%> (ø)	⬆️
autopush/web/webpush.py	100% <100%> (ø)	⬆️

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[pjenvey]

nitpick: think I've mentioned this before but we always end up w/ OneOf([onething]), should just be Equals(onething)

[jrconlin]

Fair point. I'll change the other ones that do the same thing.

[bbangert]

If the commit doesn't use a capital c for 'closes', then clog doesn't see it. Which is unfortunately kind of sad, since our contributing doc makes it sound like all lowercase 'closes' is fine. This is why some of our changelog is missing Issue linkage for what it closed.