BF+TST: bug in nifti convert for 'unknown' space

'unknown' space should work if the header matches, but if there is no
header, the zooms weren't being set, and the check was failing.  Fix and
test.
Merge pull request #294 from matthew-brett/fix-nifti-ref-no-header

BF+TST: bug in nifti convert for 'unknown' space

'unknown' space should work if the header matches, but if there is no header,
the zooms weren't being set, and the check was failing. Fix and test.