Unique scope for impressionable instance overwritten unintentionally

This is a fix for #134 where the unique conditions were unintentionally overwritten when the query was built.

The altered line is definitely a little portly and could go on a diet. However, I avoided trying to attempt to do any overhaul requiring tracking of the impressionable instance as a property to make the reference to `param[:id]` in `direct_create_statement` smarter.

:white_check_mark: Tests included and pass.