plot.imshow now obeys 'origin' kwarg.

[dcherian]

• Closes Passing origin keyword to DataArray.plot.imshow does not behave as expected #2379 (remove if there is no corresponding issue, which should only be the case for minor changes)
• Tests added (for all bug fixes or enhancements)
• Tests passed (for all non-documentation changes)
• Fully documented, including whats-new.rst for all changes and api.rst for new API (remove if this change should not be visible to users, e.g., if it is an internal clean-up, or if this is part of a larger project that will be documented later)