make `remove_index :table, :column` reversible.

This used to raise a `IrreversibleMigration` error (since #10437).
However since `remove_index :table, :column` is probably the most basic
use-case we should make it reversible again.

Conflicts:
	activerecord/CHANGELOG.md

1 files changed, 10 insertions, 6 deletions

diff --git a/activerecord/test/cases/invertible_migration_test.rb b/activerecord/test/cases/invertible_migration_test.rb
index 8144f3e5c5..99230aa3d5 100644
--- a/activerecord/test/cases/invertible_migration_test.rb
+++ b/activerecord/test/cases/invertible_migration_test.rb
@@ -144,13 +144,17 @@ module ActiveRecord
     end
 
     def test_exception_on_removing_index_without_column_option
-      RemoveIndexMigration1.new.migrate(:up)
-      migration = RemoveIndexMigration2.new
-      migration.migrate(:up)
+      index_definition = ["horses", [:name, :color]]
+      migration1 = RemoveIndexMigration1.new
+      migration1.migrate(:up)
+      assert migration1.connection.index_exists?(*index_definition)
 
-      assert_raises(IrreversibleMigration) do
-        migration.migrate(:down)
-      end
+      migration2 = RemoveIndexMigration2.new
+      migration2.migrate(:up)
+      assert_not migration2.connection.index_exists?(*index_definition)
+
+      migration2.migrate(:down)
+      assert migration2.connection.index_exists?(*index_definition)
     end
 
     def test_migrate_up