Fix difference between two date times with asFloat param

[samip77]

What does this PR do?
The difference between two date times had wrong test case which was hiding the actual issue.

The documentation for diff function mentioned:

  /// The optional [asFloat] parameter determines whether to return the
  /// difference as a floating-point number. The default value is [false].
  ///

but the asFloat default value was true

  num diff(Jiffy jiffy, {Unit unit = Unit.microsecond, bool asFloat = true}) {
    return _display.diff(dateTime, jiffy.dateTime, unit, asFloat);
  }

and in the inner function the logic was wrong

return asFloat ? _asFloor(diff) : diff;

Along with that the test case has wrong assumption, because of which it passed the test.

Issue Reference

Fixes #

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Maintenance change (non-breaking change such as upgrading a dependency, refactoring, or making a lint fix)
• Documentation update
• Breaking change (a fix or feature that would cause existing functionality to change)

Screenshots

If applicable, add screenshots to help explain your problem.

Checklist

• [ x] Wrote additional tests, if needed
• All tests have passed, you can find the scripts from the ./bin folder
• [ x] I have updated the documentation accordingly, if needed.

Additional Information

[jama5262]

@samip77 Thanks for the contribution, will take a look

[samip77]

@samip77 Thanks for the contribution, will take a look

Thanks, I had missed few test cases. updated them as well.