a date pattern like error message for a constraint causes an exception

Steps to Reproduce

1.

 <datebox constraint="after 20191017: {必須大於2019-10-17}"/>

Current Result

org.zkoss.zk.ui.UiException: Invalid constraint: after 20191017: {????2019-10-17} (the constraint should be formatted in yyyyMMdd)
at org.zkoss.zul.SimpleDateConstraint.<init>(SimpleDateConstraint.java:123)
at org.zkoss.zul.Datebox.setConstraint(Datebox.java:844)

Expected Result

no error

Debug Information

• the date string is part of the constraint error message, it's no need to match the pattern yyyyMMdd.