refactor: Change header keys to be canonical

[alexandear]

The PR enables the canonicalheader linter and changes header key strings to be in canonical form.

This is needed because canonical header keys don't need to be converted to a canonical form via http.CanonicalHeaderKey, leading to fewer allocations when adding, setting or getting a header.

Benchmarks

funcBenchmarkCanonical(b*testing.B) {
v:= http.Header{
"Canonical-Header": []string{"hello_world"},
}
b.ReportAllocs()
b.ResetTimer()
fori:=0; i<b.N; i++ {
s:=v.Get("Canonical-Header")
ifs!="hello_world" {
b.Fatal()
}
}
}
funcBenchmarkNonCanonical(b*testing.B) {
v:= http.Header{
"Canonical-Header": []string{"hello_world"},
}
b.ReportAllocs()
b.ResetTimer()
fori:=0; i<b.N; i++ {
s:=v.Get("CANONICAL-HEADER")
ifs!="hello_world" {
b.Fatal()
}
}
}

goos: darwin
goarch: arm64
pkg: github.com/lasiar/canonicalheader
cpu: Apple M1 Pro
BenchmarkNonCanonical
BenchmarkNonCanonical-8 1407914 848.2 ns/op 16 B/op 1 allocs/op
BenchmarkCanonical
BenchmarkCanonical-8 11176232 107.7 ns/op 0 B/op 0 allocs/op
PASS

[gmlewis]

Thank you, @alexandear !
LGTM.
Merging.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 92.21%. Comparing base (2b8c7fa) to head (900c040).
Report is 199 commits behind head on master.

Additional details and impacted files

@@ Coverage Diff @@## master #3389 +/- ##
==========================================
- Coverage 97.72% 92.21% -5.51% 
==========================================
Files 153 173 +20 Lines 13390 14770 +1380 ==========================================
+ Hits 13085 13620 +535 - Misses 215 1060 +845 
Partials 90 90 

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