Websocket subprotocol

[darkvariantdivine]

Fix for issue #1878, @ahopkins for your review

[ahopkins]

Thanks for this. Looks good. Just a couple small points.

Also, to fix the broken Travis build, you can run make black to fix the linting issue.

[codecov]

Codecov Report

Merging #1887 into master will increase coverage by 0.19%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #1887      +/-   ##
==========================================
+ Coverage   92.06%   92.26%   +0.19%     
==========================================
  Files          27       27              
  Lines        3089     3090       +1     
  Branches      552      553       +1     
==========================================
+ Hits         2844     2851       +7     
+ Misses        171      167       -4     
+ Partials       74       72       -2     

Impacted Files	Coverage Δ
sanic/asgi.py	93.01% <100.00%> (ø)
sanic/websocket.py	88.76% <100.00%> (+0.12%)	⬆️
sanic/server.py	80.19% <0.00%> (+1.14%)	⬆️

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[darkvariantdivine]

@ahopkins I fixed all the linter errors and made all the changes you requested :)

[ahopkins]

Looks good. Thank you.

In future, these tests can be combined into a single with @pytest-mark-parametrize. But that can be a separate PR. Let's get this in.

https://docs.pytest.org/en/latest/parametrize.html#pytest-mark-parametrize-parametrizing-test-functions