Preserve `up` and `down` return type

In Rails 4.2 calling `ActiveRecord::Migrator.migrate` would return an array of results. Without realizing that this return type was expected I accidentally introduced a change in https://github.com/rails/rails/commit/4d60e93174a3d6d90b1a06fc7515cb5cd749a6f3

This PR preserves the previous behavior and adds a test on the return type. This will need a backport to 5.0 branch.

activerecord/lib/active_record/migration.rb

@@ -1170,9 +1170,10 @@ def load_migrated
       def run_without_lock
         migration = migrations.detect { |m| m.version == @target_version }
         raise UnknownMigrationVersionError.new(@target_version) if migration.nil?
-        execute_migration_in_transaction(migration, @direction)
+        result = execute_migration_in_transaction(migration, @direction)
 
         record_environment
+        result
       end
 
       # Used for running multiple migrations up to or down to a certain value.
@@ -1181,11 +1182,12 @@ def migrate_without_lock
           raise UnknownMigrationVersionError.new(@target_version)
         end
 
-        runnable.each do |migration|
+        result = runnable.each do |migration|
           execute_migration_in_transaction(migration, @direction)
         end
 
         record_environment
+        result
       end
 
       # Stores the current environment in the database.

activerecord/test/cases/migrator_test.rb

@@ -290,6 +290,20 @@ def test_migrator_going_down_due_to_version_target
     assert_equal [[:up, 1], [:up, 2], [:up, 3]], calls
   end
 
+  def test_migrator_output
+    _, migrator = migrator_class(3)
+
+    result = migrator.migrate("valid")
+    assert_equal(3, result.count)
+
+    # Nothing migrated from duplicate run
+    result = migrator.migrate("valid")
+    assert_equal(0, result.count)
+
+    result = migrator.rollback("valid")
+    assert_equal(1, result.count)
+  end
+
   def test_migrator_rollback
     _, migrator = migrator_class(3)