MRG: change set of qform / sform into empty header

Previously we left the sform and qform empty if the affine was None, or 
matched the default affine in the header when sform and qform are empty.

From discussion at gh-182 - set the qform and sform explicitly in the case of
header=None, using the default affine, to avoid the surprise of no qform /
sform after saving, when the input affine matches the default.