GH1419 Allow Series and Index for other in Index.where(..., other)

[loicdiridollou]

• Closes Index.where(..., other) should support other with more than array-like #1419
• Tests added: Please use assert_type() to assert the type of any return value

[cmp0xff]

From the description in #1419, it seems the following tests are yet to be added:

• When the base instance is not of DatetimeIndex but just of a normal Index, .where also works with the changes
• other being a Series

Please make the tests complete. Thanks!

[loicdiridollou]

Converting to draft until response from pandas team.

[loicdiridollou]

Okay so this is allowed by pandas so I will clean the PR tonight.

[cmp0xff]

Please resolve the conflicts so that we can merge 🙂

[cmp0xff]

Thank you @loicdiridollou !