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

func BenchmarkCanonical(b *testing.B) {
	v := http.Header{
		"Canonical-Header": []string{"hello_world"},
	}
	b.ReportAllocs()
	b.ResetTimer()
	for i := 0; i < b.N; i++ {
		s := v.Get("Canonical-Header")
		if s != "hello_world" {
			b.Fatal()
		}
	}
}

func BenchmarkNonCanonical(b *testing.B) {
	v := http.Header{
		"Canonical-Header": []string{"hello_world"},
	}
	b.ReportAllocs()
	b.ResetTimer()
	for i := 0; i < b.N; i++ {
		s := v.Get("CANONICAL-HEADER")
		if s != "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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