Skip records if they are nil in preloaders_on.

Fixes #173.
Merge pull request #175 from kenips/bug/173

Skip records if they are nil in preloaders_on.
add preceding BULLET_ to env DEBUG and LOG
add inversed objects #174

impossible object is not suitable for set_inverse_instance.
replace all .id with .primary_key_value
Bumping version to 4.12.0
Support include? call on ar associations
Bumping version to 4.13.0