3D checking as already been done a few lines above through check_niimg with ensure_3d=True

Minor whitespace alignment
Fix false positive in mask -> mask_img replacement
Merge pull request #242 from lesteve/unnecessary-3d-checking-in-plot-img-with-bg

Unnecessary 3D checking in _plot_img_with_bg
Merge branch 'master' of github.com:nilearn/nilearn