elm_calendar: Fix header text bug.

elm_calendar: Fix header text bug.

Summary:
There is a bug. when calendar showing very first time.
the some of the headers(weekday name) is blank.

Because of elm_layout_text_set() called with NULL value.

Need to insert all of the weekday names before set text.

@fix

Test Plan:
Execute elementary_test
Open calendar sample.
See the all of the header names there.

Reviewers: cedric, jpeg, Hermet

Reviewed By: Hermet

Subscribers: cedric, jpeg

Differential Revision: https://phab.enlightenment.org/D4290