Fixed #22172 -- index_together now can be used with a single list

"For convenience, unique_together can be a single tuple when dealing with a single set of fields"
This convenience is missing in index_together

I think this should be there, as using double tuples/lists is not suitable.
Also changing this does not affects the behavior so much, though convenience is bettered.
I have also written the test for it.
Therefore I have opened this PR, please review it.
If it is approved, I will add the changes in the docs as well.

https://code.djangoproject.com/ticket/22172