Don't save missing close_date value as numeric zero

The value was being cast to int which which caused missing value to be interpret
as timestamp with value 0. Now value of the missing field is correctly null and
therefore not saved at all.

Fixes #5
Merge branch 'issue-5' of https://github.com/juho-jaakkola/Elgg_1.9_poll into juho-jaakkola-issue-5