Add WebSocket masking and correctly use Sec-WebSocket-Key in client

[nhooyr]

Closes #88

Thanks again for the bug report @propertyscott.

[codecov]

Codecov Report

Merging #89 into master will decrease coverage by 1%.
The diff coverage is 81.42%.

@@            Coverage Diff             @@
##           master      #89      +/-   ##
==========================================
- Coverage   79.72%   78.72%   -1.01%     
==========================================
  Files          14       14              
  Lines         957     1001      +44     
==========================================
+ Hits          763      788      +25     
- Misses        151      162      +11     
- Partials       43       51       +8

Impacted Files	Coverage Δ
dial.go	86.66% <100%> (+1.66%)	⬆️
accept.go	88.76% <100%> (+0.12%)	⬆️
websocket.go	76.29% <75.47%> (-2.5%)	⬇️

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