Mass update: flexible intervals was updated with an unchecked checkbox

How to reproduce:

• in an items list select few items
• select Mass update and click Go
• select Flexible intervals
• add new Flexible intervals
• uncheck Flexible intervals
• click Update

Debug info:

UPDATE items SET delay_flex='50/1-7,00:00-24:00', itemid='22192', flags='0', key_='trap5', hostid='10047', interfaceid=NULL WHERE (itemid IN ('22192'))
...
UPDATE items SET delay_flex='50/1-7,00:00-24:00', itemid='22193', flags='0', key_='trap6', hostid='10047', interfaceid=NULL WHERE (itemid IN ('22193'))
...