strconv: fix handling of BOMs in CanBackquote

A byte order mark  (BOM) cannot be backquoted.

LGTM=r
R=golang-codereviews, gobot, r
CC=golang-codereviews
https://golang.org/cl/112310043

src/pkg/strconv/quote.go

@@ -147,7 +147,10 @@ func CanBackquote(s string) bool {
 		r, wid := utf8.DecodeRuneInString(s)
 		s = s[wid:]
 		if wid > 1 {
-			continue // All multibyte runes are correctly encoded and assumed printable.
+			if r == '\ufeff' {
+				return false // BOMs are invisible and should not be quoted.
+			}
+			continue // All other multibyte runes are correctly encoded and assumed printable.
 		}
 		if r == utf8.RuneError {
 			return false

src/pkg/strconv/quote_test.go

@@ -148,8 +148,8 @@ var canbackquotetests = []canBackquoteTest{
 	{`☺`, true},
 	{"\x80", false},
 	{"a\xe0\xa0z", false},
-	{"\ufeffabc", true},
-	{"a\ufeffz", true},
+	{"\ufeffabc", false},
+	{"a\ufeffz", false},
 }
 
 func TestCanBackquote(t *testing.T) {