astropy.wcs failing to recognize invalid region in ZPN and HPX projections

In the following header:

```
NAXIS   = 2
NAXIS1  =                 200
NAXIS2  =                 200
CTYPE1  = 'RA---ZPN'
CRPIX1  =  100
CDELT1  =  -6.666666666667E-02
CRVAL1  =   0.000000000000E+00
CTYPE2  = 'DEC--ZPN'
CRPIX2  =   100
CDELT2  =   6.666666666667E-02
CRVAL2  =  -9.000000000000E+01
LONPOLE =   1.800000000000E+02 / Native longitude of celestial pole
LATPOLE =  -9.000000000000E+01 / Native latitude  of celestial pole
PV2_0   =  10.000000000000E-02 / Projection parameter 0
PV2_1   =   9.750000000000E-01 / Projection parameter 1
```
much of the central part of the image should have no valid world coordinates (according to ds9 for example) but ``astropy.wcs`` does not seem to give ``np.nan`` as might be expected. For example, slightly off from the center:

```
In [18]: wcs.wcs_pix2world(110,110, 0)
Out[18]: [array(134.99999999999994), array(-85.18719178916112)]
```

whereas ds9 does not report any valid coordinates for that pixel position.

cc @mdboom