[1.6.x] Fixed #21410 -- prefetch_related() for ForeignKeys with related_name='+'

Regression introduced by commit 9777442.

Thanks to trac username troygrosfield for the report and test case.

Backpatch of cb83448891f2920c926f61826d8eae4051a3d8f2 from master.

Conflicts:

	tests/prefetch_related/models.py
[1.6.x] Added release note for #21410; thanks Loic.

Backport of 94d567ba30 from master