net/http: avoid some allocations in DetectContentType

Change-Id: I64985f8de7ca09e63208e8c949a5d4f4fc09073f
Reviewed-on: https://go-review.googlesource.com/1230Reviewed-by: David Symonds <dsymonds@golang.org>

src/net/http/sniff.go

@@ -38,7 +38,11 @@ func DetectContentType(data []byte) string {
 }
 
 func isWS(b byte) bool {
-	return bytes.IndexByte([]byte("\t\n\x0C\r "), b) != -1
+	switch b {
+	case '\t', '\n', '\x0c', '\r', ' ':
+		return true
+	}
+	return false
 }
 
 type sniffSig interface {
@@ -161,6 +165,8 @@ func (h htmlSig) match(data []byte, firstNonWS int) string {
 	return "text/html; charset=utf-8"
 }
 
+var mp4ftype = []byte("ftyp")
+
 type mp4Sig int
 
 func (mp4Sig) match(data []byte, firstNonWS int) string {
@@ -172,7 +178,7 @@ func (mp4Sig) match(data []byte, firstNonWS int) string {
 	if boxSize%4 != 0 || len(data) < boxSize {
 		return ""
 	}
-	if !bytes.Equal(data[4:8], []byte("ftyp")) {
+	if !bytes.Equal(data[4:8], mp4ftype) {
 		return ""
 	}
 	for st := 8; st < boxSize; st += 4 {