fix: fix expiry for `to_json()` (#589)
* This patch for </issues/501> includes the following fixes:
- The access token is always set to `None`, so the fix involves using (the access) `token` from the saved JSON credentials file.
- For refresh needs, `expiry` also needs to be saved via `to_json()`.
- DUMP: As `expiry` is a `datetime.datetime` object, serialize to `datetime.isoformat()` in the same [`oauth2client` format](https://github.com/googleapis/oauth2client/blob/master/oauth2client/client.py#L55) for consistency.
- LOAD: Add code to restore `expiry` back to `datetime.datetime` object when imported.
- LOAD: If `expiry` was unsaved, automatically set it as expired so refresh takes place.
- Minor `scopes` updates
- DUMP: Add property for `scopes` so `to_json()` can grab it
- LOAD: `scopes` may be saved as a string instead of a JSON array (Python list), so ensure it is Sequence[str] when imported.
2 files changed
