article metadata updated_at and mimetype missing - used by KoReader

[clach04]

A few entries are missing but article.updated_at is use by wallabag plugin for KoReader.

https://github.com/clach04/wallabag2.koplugin/blob/c7fd203be1723a633e7300cd1b718438e45861f3/wallabag2.koplugin/main.lua#L603

As is mimetype https://github.com/clach04/wallabag2.koplugin/blob/c7fd203be1723a633e7300cd1b718438e45861f3/wallabag2.koplugin/main.lua#L580

Plugin runs fine if values are missing but should implement.

• https://github.com/search?q=repo%3Awallabag%2Fwallabag%20updated_at&type=code
• https://github.com/wallabag/wallabag/blob/c2bfc0f35966e91313e547bbd83036fb607b6566/tests/Wallabag/ImportBundle/fixtures/wallabag-v2-read.json#L8

[clach04]

Adding mimetype will reduce code path in KoReader plugin. Hilariously add updated timestamp will increase code path followed in the plugin.