Cube header not updated in write to file

the header of Cube objects is not updated when written to file

---

scube=Cube('./subcubeOII_DATACUBE_FINAL_ZAP_J1422_QSO_subtracted.fits')

In [67]: pyfits.open(scube.filename)[1].header['BUNIT']
Out[67]: '10**(-20)*erg/s/cm**2/Angstrom'

In [68]: scube.data_header.set('BUNIT','10**(-20)*erg/s/cm**2')

In [69]: scube.data_header['BUNIT']
Out[69]: '10**(-20)*erg/s/cm**2'

In [70]: rm tmp.fits
rm: remove regular file ‘tmp.fits’? y

In [71]: scube.write('tmp.fits')

In [73]: pyfits.open('tmp.fits')[1].header['BUNIT']
Out[73]: '10**(-20)*erg/s/cm**2/Angstrom'