time: Fix missing colon when formatting time zone offsets with seconds

When formatting time zone offsets with seconds using the stdISO8601Colon
and stdNumColon layouts, the colon was missing between the hour and minute
parts.

Fixes #8497.

LGTM=r
R=golang-codereviews, iant, gobot, r
CC=golang-codereviews
https://golang.org/cl/126840043

src/pkg/time/format.go

@@ -556,7 +556,7 @@ func (t Time) Format(layout string) string {
 				b = append(b, '+')
 			}
 			b = appendUint(b, uint(zone/60), '0')
-			if std == stdISO8601ColonTZ || std == stdNumColonTZ {
+			if std == stdISO8601ColonTZ || std == stdNumColonTZ || std == stdISO8601ColonSecondsTZ || std == stdNumColonSecondsTZ {
 				b = append(b, ':')
 			}
 			b = appendUint(b, uint(zone%60), '0')

src/pkg/time/format_test.go

@@ -502,10 +502,11 @@ func TestParseSecondsInTimeZone(t *testing.T) {
 }
 
 func TestFormatSecondsInTimeZone(t *testing.T) {
-	d := Date(1871, 9, 17, 20, 4, 26, 0, FixedZone("LMT", -(34*60+8)))
-	timestr := d.Format("2006-01-02T15:04:05Z070000")
-	expected := "1871-09-17T20:04:26-003408"
-	if timestr != expected {
-		t.Errorf("Got %s, want %s", timestr, expected)
+	for _, test := range secondsTimeZoneOffsetTests {
+		d := Date(1871, 1, 1, 5, 33, 2, 0, FixedZone("LMT", test.expectedoffset))
+		timestr := d.Format(test.format)
+		if timestr != test.value {
+			t.Errorf("Format = %s, want %s", timestr, test.value)
+		}
 	}
 }