Fix a false positive for unindexed_foreign_keys when having foreign key on a primary key column

[fatkodima]

Fixes #156.

[jdufresne]

@fatkodima I discovered the same false positive exists for missing_unique_indexes.

add a unique index on children(parent_id) - using `has_one` in Parent without an index can lead to duplicates

The primary key index is already unique and so doesn't require an additional one.

Would you prefer I file a new issue or is this comment here sufficient?

[fatkodima]

This comment is sufficient, thanks.