New WCS TPV support doesn't work with DECAM images #3904

Sorry to say this, @mdboom - but I just attempted the procedure I described in my comment in #3845, and it still doesn't seem to work even after #3878 . I've copied my procedure below, although not a critical change: it seem to work with neither all_pix2world nor wcs_pix2world now...

1. Go to http://www.portal-nvo.noao.edu/search/query and select "CTIO 4m + DECam imager" under Telescope&Instrument, and check "calibrated images" for "Mosaic, NEWFIRM, and DECam". If you want exactly the file I tested on, you'll also want to choose Program number="2012B-0001", but I don't think that actually matters
2. Hit "search", and then find the first result the search returns that has "Product"="image". Hit "retreive" on that row and it will download a DECam image.
3. You should see this with that image:

>>> f = fits.open('c4d_130831_235507_ooi_g_d3.fits.fz')
>>> w = wcs.WCS(f[1].header)
>>> ra, dec = w.all_pix2world([0,10, 15],[0,10, 20],0)
>>> ra
array([ 301.2065433,  301.2065433,  301.2065433])
>>> dec
array([-50.93399553, -50.93399553, -50.93399553])

which is clearly wrong: they should be different ra/dec.
4. Also, I see this:

>>> ra, dec = w.wcs_pix2world([0,10, 15],[0,10, 20],0)
>>> ra
array([ 301.2065433,  301.2065433,  301.2065433])
>>> dec
array([-50.93399553, -50.93399553, -50.93399553])

which is the same incorrect answer, as opposed to when I tried this in #3845, where wcs_pix2world did work.