Add quoteString function.

This is to just quote a string s -> "s"

parse_test.go

@@ -834,8 +834,8 @@ func TestTxtEqual(t *testing.T) {
 	rr1.Txt = []string{"a\"a", "\"", "b"}
 	rr2, _ := NewRR(rr1.String())
 	if rr1.String() != rr2.String() {
-//		t.Fail() // Is this a bug?
-		t.Logf("Two TXT records should match")
+		t.Fail()
+		t.Logf("These two TXT records should match")
 		t.Logf("\n%s\n%s\n", rr1.String(), rr2.String())
 	}
 }

types.go

@@ -1502,6 +1502,10 @@ func euiToString(eui uint64, bits int) (hex string) {
 	return
 }
 
+func quoteString(s string) string {
+	return "\"" + s +  "\""
+}
+
 // Map of constructors for each RR wire type.
 var rr_mk = map[uint16]func() RR{
 	TypeA:          func() RR { return new(A) },