Wrapped resource.serialize with json.dumps() to convert object to json

[evmende]

Fixes #356

example PUT to api/v1/resources/1:

{
    "notes": "test2"
}

old logs:

2020-08-13 02:54:24,753 INFO User: ekmendes1@gmail.com Route: /api/v1/resources/1 Payload: {'notes': 'test2'}
2020-08-13 02:54:24,788 INFO Updating resource. Old data: {'id': 1, 'name': 'Free Programming Books', 'url': 'https://github.com/vhf/free-programming-books', 'category': 'Books', 'languages': ['multiple'], 'paid': False, 'notes': 'test1', 'upvotes': 0, 'downvotes': 0, 'times_clicked': 0, 'created_at': '2020-08-02 03:04:21', 'last_updated': '2020-08-13 02:49:18'}

new logs:

2020-08-13 02:49:18,768 INFO User: ekmendes1@gmail.com Route: /api/v1/resources/1 Payload: {'notes': 'test1'}
2020-08-13 02:49:18,816 INFO Updating resource. Old data: {"id": 1, "name": "Free Programming Books", "url": "https://github.com/vhf/free-programming-books", "category": "Books", "languages": ["multiple"], "paid": false, "notes": null, "upvotes": 0, "downvotes": 0, "times_clicked": 0, "created_at": "2020-08-02 03:04:21", "last_updated": ""}

[aaron-junot]

LGTM