Fix index for datetime64 conversion. Fixes #13937
- [x] closes #13937
- [x] tests added / passed
- [x] passes `git diff upstream/master | flake8 --diff`
- [x] 0.20.0

Fix index for datetime64 conversion

This PR fixes index for datetime64 conversion. Also,
update tests - pandas/pandas/tests/tests_convert_to.py

Fixes https://github.com/pydata/pandas/issues/13937
