Fixed an issue with task configuration rewriting in the .add_job method

[HK-Mattew]

Changes

Fixes

• Some changes were made to the .configure_task method to fix an issue mentioned in Add .get_task method to Schedulers - APS v4 #953

Checklist

If this is a user-facing code change, like a bugfix or a new feature, please ensure that
you've fulfilled the following conditions (where applicable):

• You've added tests (in tests/) added which would fail without your patch
• You've updated the documentation (in docs/, in case of behavior changes or new
  features)
• You've added a new changelog entry (in docs/versionhistory.rst).

If this is a trivial change, like a typo fix or a code reformatting, then you can ignore
these instructions.

Updating the changelog

If there are no entries after the last release, use **UNRELEASED** as the version.
If, say, your patch fixes issue #999, the entry should look like this:

* Fix big bad boo-boo in the async scheduler (#999 <https://github.com/agronholm/apscheduler/issues/999>_; PR by @yourgithubaccount)

If there's no issue linked, just link to your pull request instead by updating the
changelog after you've created the PR.

[HK-Mattew]

If these changes are accepted, let me know and I'll add the entry to the changelog.

[coveralls]

coverage: 92.224% (+0.2%) from 92.006%
when pulling 3f3b931 on HK-Mattew:master
into ec80485 on agronholm:master.