Fix error in ActiveRecord::Migration.check_pending!

Since rails@e9d835f this method has not worked, it has failed with this error:

```
NoMethodError: undefined method `pending_migrations' for #<ActiveRecord::MigrationContext:0x00005598468f8e58>
Did you mean?  pending_migration_versions
```

I agree with Eileen that eventually we could deprecate it, but to get it working I just reversed the changes in rails@e9d835f and added a regression test.

Author: ghiculescu

activerecord/lib/active_record/migration.rb

@@ -644,9 +644,7 @@ def nearest_delegate # :nodoc:
 
       # Raises <tt>ActiveRecord::PendingMigrationError</tt> error if any migrations are pending.
       def check_pending!(connection = ActiveRecord::Tasks::DatabaseTasks.migration_connection)
-        if pending_migrations = connection.migration_context.pending_migrations
-          raise ActiveRecord::PendingMigrationError.new(pending_migrations: pending_migrations)
-        end
+        raise ActiveRecord::PendingMigrationError if connection.migration_context.needs_migration?
       end
 
       def load_schema_if_pending!

activerecord/test/cases/migration/pending_migrations_test.rb

@@ -89,6 +89,10 @@ def test_with_stdlib_logger
         private
           def assert_pending_migrations(*expected_migrations)
             2.times do
+              assert_raises ActiveRecord::PendingMigrationError do
+                ActiveRecord::Migration.check_pending!
+              end
+
               error = assert_raises ActiveRecord::PendingMigrationError do
                 CheckPending.new(proc { flunk }).call({})
               end
@@ -105,6 +109,10 @@ def assert_no_pending_migrations
             check_pending = CheckPending.new(app)
 
             2.times do
+              assert_nothing_raised do
+                ActiveRecord::Migration.check_pending!
+              end
+
               app.expect :call, nil, [{}]
               check_pending.call({})
               app.verify