Activity Panel: Inbox: Avoid Notice on Updating a Note: Convert WC_DateTime to timestamps

[allendav]

Fixes #786

I swear I already fixed this when I saw it during development, but apparently I didn't :)

Screenshots

Detailed test instructions:

• WP_DEBUG true
• Install and activate the second activity panel example
• Make sure you do NOT see any notice in your PHP log, and that mysql shows the note content updating on refreshing any wp-admin page, e.g.

mysql> select * from wcz_woocommerce_admin_notes;
+---------+-------------------------+------+--------+--------------------------+----------------------------------+------+-------------------------+------------+-------------------------+---------------------+---------------+
| note_id | name                    | type | locale | title                    | content                          | icon | content_data            | status     | source                  | date_created        | date_reminder |
+---------+-------------------------+------+--------+--------------------------+----------------------------------+------+-------------------------+------------+-------------------------+---------------------+---------------+
|       2 | wapi-example-plugin-two | info | en_US  | Domain Renewal Coming Up | Your domain expires in 254 days. | info | {"expires_in_days":254} | unactioned | wapi-example-plugin-two | 2018-11-06 21:52:07 | NULL          |
+---------+-------------------------+------+--------+--------------------------+----------------------------------+------+-------------------------+------------+-------------------------+---------------------+---------------+
1 row in set (0.00 sec)

mysql> select * from wcz_woocommerce_admin_notes;
+---------+-------------------------+------+--------+--------------------------+----------------------------------+------+-------------------------+------------+-------------------------+---------------------+---------------+
| note_id | name                    | type | locale | title                    | content                          | icon | content_data            | status     | source                  | date_created        | date_reminder |
+---------+-------------------------+------+--------+--------------------------+----------------------------------+------+-------------------------+------------+-------------------------+---------------------+---------------+
|       2 | wapi-example-plugin-two | info | en_US  | Domain Renewal Coming Up | Your domain expires in 265 days. | info | {"expires_in_days":265} | unactioned | wapi-example-plugin-two | 2018-11-06 21:52:07 | NULL          |
+---------+-------------------------+------+--------+--------------------------+----------------------------------+------+-------------------------+------------+-------------------------+---------------------+---------------+
1 row in set (0.00 sec)

[allendav]

Nevermind. I just noticed I already fixed this in add/admin-notices-wc-helper