add a spec example for supports_partial_indexes?
add a spec example for supports_partial_indexes?

(with some refactoring for DRY migration.suppress_messages and migration.remove_index)

postgresql_adapter is now at 100% coverage, and abstract_adapter is a little more covered than before.
PeerStreet/schema_plus,allenwq/schema_validations,masarakki/schema_plus,Tretti/schema_plus,SchemaPlus/schema_plus