Fix article serialization for images

[MaxDall]

There was a bug when serializing the images attribute described in #702. This PR provides a quick fix so that the to_json method also works for images.

[addie9800]

I am having some issues with this.

from fundus import PublisherCollection, Crawler,
from tests.utility import get_test_articles

publisher = PublisherCollection.de.Tagesschau
crawler = Crawler(publisher)

if __name__ == "__main__":
    for article in crawler.crawl(max_articles=1, only_complete=False, error_handling='suppress', save_to_file="E:\\Temp\\test.json"):
        print(article)

Running this code gives me an error: TypeError: Attribute 'topics' of type <class 'lxml.etree._ElementUnicodeResult'> is not JSON serializable. This can be easily fixed by modifying your code slightly:

        def serialize(v: Any) -> JSONVal:
            if hasattr(v, "serialize"):
                return v.serialize()  # type: ignore[no-any-return]
            elif isinstance(v, datetime):
                return str(v)
            elif isinstance(v, str):
                return v
            raise TypeError(f"Attribute {attribute!r} of type {type(v)!r} is not JSON serializable")

Yet, this causes this error: line 342, in <lambda> json.dumps(crawled_articles, default=lambda o: o.to_json(), ensure_ascii=False, indent=4) AttributeError: 'datetime.datetime' object has no attribute 'to_json' which causes me to scratch my head. I have played around with it for some time now, but can't lay my finger on the issue. Perhaps you see can see it.