Fixed customfields checkbox default value handling

This patch tries to fix the following situation:

Currently it is not possible to set all of the checkboxes of a custom
fields to an unchecked state, if the default values for them are not
empty. Trying to do so, will cause the checkboxes to fallback to their
default value.
Merge pull request #107 from decden/patch-checkbox-default-values

Fixed customfields checkbox default value handling