fix(dumpkit): JSON output for test.pcapng does not parse #1152

Describe the bug
With the default engine, extract(fin='test.pcapng', format='json') writes JSON that does not parse: Expecting ':' delimiter: line 1019 column 12. Because PyShark falls back to the default engine, it is affected too. The behaviour is the same before and after #1149.

Reproduction
Generate test.pcapng with make_samples.py, extract it to JSON, then json.load the output.

Expected behavior
The output file is valid JSON.

Additional context
Found by the #1104 worker. The cause may be in dictdumper, a dependency, or in pcapkit's own handling of the dumped values. That needs checking first.