Improve test coverage

[nhooyr]

With Cookie and CloseError unit tests

[codecov]

Codecov Report

Merging #53 into master will increase coverage by 0.24%.
The diff coverage is 78.26%.

@@            Coverage Diff             @@
##           master      #53      +/-   ##
==========================================
+ Coverage   75.98%   76.22%   +0.24%     
==========================================
  Files          11       11              
  Lines         662      673      +11     
==========================================
+ Hits          503      513      +10     
- Misses        131      134       +3     
+ Partials       28       26       -2

Impacted Files	Coverage Δ
header.go	94.44% <ø> (ø)	⬆️
accept.go	87.36% <0%> (ø)	⬆️
statuscode_string.go	25.92% <0%> (ø)	⬆️
websocket.go	76.72% <72.72%> (-4.14%)	⬇️
statuscode.go	88.23% <90.32%> (+20.23%)	⬆️
dial.go	81.94% <0%> (+5.55%)	⬆️
opcode_string.go	23.52% <0%> (+23.52%)	⬆️

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