elm: fix backward compatibility for "default" name part

elm: fix backward compatibility for "default" name part

Summary:
If you use legacy content/text set/get/unset with "default" part name,
_elm_layout_part_aliasing_eval change the part name to "elm.swallow.content".
Because internal _content_set/get/unset checked only "default" string,
legacy content/test set/get/unset with "default" part name was not working.
This patch fixes that bug.

Test Plan: elm_object_part_content/text_set(obj, "default", content)

Reviewers: Jaehyun_Cho, woohyun

Reviewed By: Jaehyun_Cho

Subscribers: cedric, zmike

Tags: #efl

Differential Revision: https://phab.enlightenment.org/D6185