RF: make header update only if changed affine

We were having trouble with the header updating on load of an image, and
changing the underlying header information.

https://github.com/nipy/nibabel/pull/164

Use the Analyze trick to only update the header if the affine has
changed.
BF: with new save affine algorithm, zooms=1

No we are not resetting the deltas on loading the image, we should get
zooms of 1 from the test image.
Merge pull request #1 from matthew-brett/setting-mgh-affines

Setting mgh affines