Add test for check_niimg with ensure_3d=True and non-3d image as argument

Fix missing space and typo in error message
PEP8 tweak while I was touching the file
Add missing space
Merge pull request #243 from lesteve/add-test-for-check-niimg-with-non-3d-image

Add test for check_niimg with ensure_3d=True and a non 3d image as argument