Modifications to DF_Model_result->populate()

What I did

• Now returns the result object for chaining.
  • This is useful for chained calls like $my_thing = $this->thing_model->get()->populate($data)->save();
• Declared the parameter type as array, and changed default value to an empty array.

Implications

None

Setup

None

How to test

• Code review.
• Test with a call like $my_thing = $this->thing_model->get()->populate($data)->save();
  • Confirm that the updated object is saved to the database.