update: Ensure method _asdict also converts Word object into a native dict

[klebster2]

Currently, after segments are generated with the WhiperModel.transcribe() method and word_timestamps=True, upon using method Segment()._asdict, we get output like the following:

In the image above, the upper red box is undesired, because _asdict should convert to native types. Additionally, this causes information, such as the dictionary keys are also lost when we convert the object that is supposedly a dict using json.dumps as exemplified in the lower red box.

The change presented therefore ensures that all Word() objects are first converted to native types, so that the keys for each Word() are retained when the _asdict method is called, or json.dumps(segment._asdict()) is called.