Don't use first_or_create to merge seed data with existing dataset

[jopolsky]

** Execute seed:school_districts
seeding school districts (19059 rows)
rake aborted!
ActiveRecord::RecordNotUnique: Mysql2::Error: Duplicate entry '100021' for key 'PRIMARY': INSERT INTO `school_districts` (`id`, `name`, `city`, `state`, `zip`, `created_at`, `updated_at`) VALUES (100021, 'BREWERS PORCH CHILDRENS CENTER', 'TUSCALOOSA', 'AL', '35487', '2017-10-17 15:30:27', '2017-10-17 15:30:27')

Seeding calls find_or_create_all_from_tsv which was executing SchoolDistrict.where(attributes).first_or_create! on the model. This doesn't support the case where existing data has changed. For instance, a school district entry in the TSV file where the city name has changed.

I created a new method for #18396 which merges the results. This change forwards the find_or_create_all_from_tsv call to the new merge_from_csv.

[wjordan]

LGTM, given that you ran these changes locally and merges the results properly.