Return from to_timedelta is forced to dtype timedelta64[ns]. 

This is a revised change, fixing the issue I experienced (issue #9011). The proper change seems to be setting the dtype to ```'timedelta64[ns]'```, as was already done for the other cases in the ```to_timedelta``` routine. 

I have also added more tests, and moved them to the ```test_timedeltas``` function (next to the other tests of the functionality of the constructor method). I have not looked too much at #8886, it is difficult for me to understand the issue.