Honor primary_key option in $belongs_to

Fixes primary_key support when using belongs_to relationship (#364).
Honor primary_key option in $has_many when eager loading.

Fixes primary_key support when using has_many relationship and eager loading together (#366).
Merge branch '366-has-many-eager-primary-key' of https://github.com/tuupola/php-activerecord
Merge branch '364-belongs-to-primary-key' of https://github.com/tuupola/php-activerecord