WCS changes velocity units from km/s to m/s

`astropy.wcs` converts `km/s` to `m/s`.  This is somewhat new behavior and highly unexpected.  For all I know, it may be part of a FITS convention, but such a convention was certainly ignored by many packages and is highly inconvenient.

```
In [19]: h = fits.Header().fromtextfile('w51_co_h2co_cube_grid.hdr')

In [20]: h
Out[20]:
SIMPLE  =                    T / conforms to FITS standard
BITPIX  =                  -64 / array data type
NAXIS   =                    3 / number of array dimensions
NAXIS1  =                  400
NAXIS2  =                  300
NAXIS3  =                  251
EQUINOX =               2000.0
CTYPE1  = 'GLON-CAR'
CTYPE2  = 'GLAT-CAR'
CTYPE3  = 'VRAD'
SPECSYS = 'LSRK'
CUNIT3  = 'km/s    '
BUNIT   = 'K       '
CRVAL1  =            49.209553
CRVAL2  =                  0.0
CRVAL3  =                 50.0
CRPIX1  =                200.0
CRPIX2  =                288.0
CRPIX3  =                125.0
CDELT1  = -0.00333333333333333
CDELT2  = 0.003333333333333333
CDELT3  =                  0.5
RESTFREQ=                1000.
TELESCOP= 'Arecibo'

In [21]: w = wcs.WCS(h)

In [22]: w.wcs.cunit
Out[22]: ['deg', 'deg', 'm/s']

In [25]: astropy.__version__
Out[25]: '0.4.dev6828'
```

`w.wcs.cunit` should be `km/s`, since `CUNIT3` was `'km/s'`.  Note that it actually does the conversion of other header units, so the header isn't *wrong*, it's just not what I want.