mime: remove allocation introduced in recent fix

CL 150417 was submitted before I could recommend this change to remove
an unnecessary allocation.

Updates #28849

Change-Id: I4cd655f62bb3d00eda6c997f074785385bceee0c
Reviewed-on: https://go-review.googlesource.com/c/150498Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/mime/mediatype.go

@@ -56,7 +56,8 @@ func FormatMediaType(t string, param map[string]string) string {
 
 		b.WriteByte('"')
 		offset := 0
-		for index, character := range []byte(value) {
+		for index := 0; index < len(value); index++ {
+			character := value[index]
 			if character == '"' || character == '\\' {
 				b.WriteString(value[offset:index])
 				offset = index