code: func canonicalizeHeaderKey(k []byte) {
  	upper := true
  	for i, c := range k {
  		if upper && 'a' <= c && c <= 'z' {
  			k[i] &= toUpper
  		} else if !upper && 'A' <= c && c <= 'Z' {
  			k[i] |= toLower
  		}
  		upper = c == '-'
  	}
  }
docs: // Algorithm below is like standard textproto/CanonicalMIMEHeaderKey, except
  // that it operates with slice of bytes and modifies it inplace without copying.