Refactored ActiveRecord::find() so that find(null) will also return an ActiveRecord instance instead of ActiveQuery.

issue #2892

so information about changed values is available in `afterSave`-event.

fixes #2892

this problem occurs when joining active record but not selecting the
primary key field. records get populated but they do not have primary
key value populated which results in only one record left over in the
result.

issue #2852

Fixes #2726: ActiveRecord now fills default values on creating new instance of the model if defaults are available from DB schema

Changed comment again to actually be correct. This is also consistent with the draft of the definitive guide.

Modified example in DataReader usage comment to reference a DB Command function which accepts a SQL string argument.

Changed comment again to actually be correct. This is also consistent with the draft of the definitive guide.

Modified example in DataReader usage comment to reference a DB Command function which accepts a SQL string argument.

Fixes #2571: Fixed the bug that batchInsert will fail for SQLite if the values contain null or boolean false;
Fixes #2683: Fixed the bug that batchInsert will fail for MySQL if the values contain boolean false.