net/url: add upperhex const instead of using string literal

The mime and strconv packages already have a const with this name & value.

Change-Id: Ibd7837f854ac8ec3f57943a9d1db07f4cf6db858
GitHub-Last-Rev: 775cdce3b75350aa3b9a6f31f04cfdd0033e9ac3
GitHub-Pull-Request: golang/go#34389
Reviewed-on: https://go-review.googlesource.com/c/go/+/196437Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/net/url/url.go

@@ -42,6 +42,8 @@ func (e *Error) Temporary() bool {
 	return ok && t.Temporary()
 }
 
+const upperhex = "0123456789ABCDEF"
+
 func ishex(c byte) bool {
 	switch {
 	case '0' <= c && c <= '9':
@@ -324,8 +326,8 @@ func escape(s string, mode encoding) string {
 			j++
 		case shouldEscape(c, mode):
 			t[j] = '%'
-			t[j+1] = "0123456789ABCDEF"[c>>4]
-			t[j+2] = "0123456789ABCDEF"[c&15]
+			t[j+1] = upperhex[c>>4]
+			t[j+2] = upperhex[c&15]
 			j += 3
 		default:
 			t[j] = s[i]