Permit VALUE=DATE parameter when parsing EXDATE and RDATE

This parsing results to the first second of the day specified by date

str.go

@@ -360,7 +360,7 @@ func StrToDatesInLoc(str string, defaultLoc *time.Location) (ts []time.Time, err
 		for _, param := range params {
 			if strings.HasPrefix(param, "TZID=") {
 				loc, err = parseTZID(param)
-			} else if param != "VALUE=DATE-TIME" {
+			} else if param != "VALUE=DATE-TIME" && param != "VALUE=DATE" {
 				err = fmt.Errorf("unsupported: %v", param)
 			}
 			if err != nil {

str_test.go

@@ -321,6 +321,20 @@ func TestSetParseLocalTimes(t *testing.T) {
 	})
 }
 
+func TestRDateValueDateStr(t *testing.T) {
+	input := []string{
+		"RDATE;VALUE=DATE:20180223",
+	}
+	s, err := StrSliceToRRuleSet(input)
+	if err != nil {
+		t.Error(err)
+	}
+	d := s.GetRDate()[0]
+	if !d.Equal(time.Date(2018, 02, 23, 0, 0, 0, 0, time.UTC)) {
+		t.Error("Bad time parsed: ", d)
+	}
+}
+
 // Helper for TestRFCSetStr and TestSetStr
 func assertRulesMatch(set *Set, t *testing.T) {
 	// matching parsed RRules