[nit] reduce alloc on commonHeader initialization

bench_result.txt

# go test -v -bench=. | prettybench
PASS
benchmark             iter       time/iter   bytes alloc        allocs
---------             ----       ---------   -----------        ------
BenchmarkBefore-4   200000   7609.00 ns/op     4427 B/op   4 allocs/op
BenchmarkAfter-4    500000   3633.00 ns/op     2414 B/op   1 allocs/op

ch_test.go

package ch

import (
	"testing"
)

func before() {
	// commonHeader interns common header strings.
	var commonHeader = make(map[string]string)
	for _, v := range []string{
		"Accept",
		"Accept-Charset",
		"Accept-Encoding",
		"Accept-Language",
		"Accept-Ranges",
		"Cache-Control",
		"Cc",
		"Connection",
		"Content-Id",
		"Content-Language",
		"Content-Length",
		"Content-Transfer-Encoding",
		"Content-Type",
		"Cookie",
		"Date",
		"Dkim-Signature",
		"Etag",
		"Expires",
		"From",
		"Host",
		"If-Modified-Since",
		"If-None-Match",
		"In-Reply-To",
		"Last-Modified",
		"Location",
		"Message-Id",
		"Mime-Version",
		"Pragma",
		"Received",
		"Return-Path",
		"Server",
		"Set-Cookie",
		"Subject",
		"To",
		"User-Agent",
		"Via",
		"X-Forwarded-For",
		"X-Imforwards",
		"X-Powered-By",
	} {
		commonHeader[v] = v
	}
	_ = commonHeader
}

func after() {
	// commonHeader interns common header strings.
	var commonHeader map[string]string
	vs := [...]string{
		"Accept",
		"Accept-Charset",
		"Accept-Encoding",
		"Accept-Language",
		"Accept-Ranges",
		"Cache-Control",
		"Cc",
		"Connection",
		"Content-Id",
		"Content-Language",
		"Content-Length",
		"Content-Transfer-Encoding",
		"Content-Type",
		"Cookie",
		"Date",
		"Dkim-Signature",
		"Etag",
		"Expires",
		"From",
		"Host",
		"If-Modified-Since",
		"If-None-Match",
		"In-Reply-To",
		"Last-Modified",
		"Location",
		"Message-Id",
		"Mime-Version",
		"Pragma",
		"Received",
		"Return-Path",
		"Server",
		"Set-Cookie",
		"Subject",
		"To",
		"User-Agent",
		"Via",
		"X-Forwarded-For",
		"X-Imforwards",
		"X-Powered-By",
	}
	commonHeader = make(map[string]string, len(vs))
	for _, v := range vs {
		commonHeader[v] = v
	}
	_ = commonHeader
}

func BenchmarkBefore(b *testing.B) {
	b.ReportAllocs()
	for i := 0; i < b.N; i++ {
		before()
	}
}

func BenchmarkAfter(b *testing.B) {
	b.ReportAllocs()
	for i := 0; i < b.N; i++ {
		after()
	}
}

patch.diff

diff --git a/src/net/textproto/reader.go b/src/net/textproto/reader.go
index e07d1d6..464e027 100644
--- a/src/net/textproto/reader.go
+++ b/src/net/textproto/reader.go
@@ -631,10 +631,10 @@ func canonicalMIMEHeaderKey(a []byte) string {
 }
 
 // commonHeader interns common header strings.
-var commonHeader = make(map[string]string)
+var commonHeader map[string]string
 
 func init() {
-	for _, v := range []string{
+	vs := [...]string{
 		"Accept",
 		"Accept-Charset",
 		"Accept-Encoding",
@@ -674,7 +674,9 @@ func init() {
 		"X-Forwarded-For",
 		"X-Imforwards",
 		"X-Powered-By",
-	} {
+	}
+	commonHeader = make(map[string]string, len(vs))
+	for _, v := range vs {
 		commonHeader[v] = v
 	}
 }