Fix JSON Feed parser: use correct key for date_updated and prefer content_html

[danishashko]

Two bugs in the JSON Feed parser:

1. KeyError when parsing date_updated

parse_entry() checks "date_updated" in e (correct per the JSON Feed spec) but then reads e["date_modified"] which doesn't exist in JSON Feed. Every entry with a date_updated field raises a KeyError.

# current (broken):
if "date_updated" in e:
    entry["updated"] = e["date_modified"]        # KeyError
    entry["updated_parsed"] = _parse_date(e["date_modified"])  # KeyError

# fixed:
if "date_updated" in e:
    entry["updated"] = e["date_updated"]
    entry["updated_parsed"] = _parse_date(e["date_updated"])

2. content_text silently wins over content_html

The current if/elif returns content_text whenever present, even if content_html is also present. The JSON Feed 1.1 spec recommends HTML-capable clients prefer content_html. Swapping the branches gives richer content by default and matches the spec intent.

Both bugs are in feedparser/parsers/json.py, no new imports needed.