WCS conversion functions accept 2xN and Nx2 arrays

From the documentation, it looks like (for 2 dimensions) the conversion routines should accept a 2xN array, but it seems that both are accepted without error and produce different results:

```
In [7]: w.wcs_world2pix(np.random.random((2,3)),1)
Out[7]: 
array([[  5.30032953e+005,   1.29954708e+004,   2.14987419e-314],
       [  6.98357305e+005,   1.83455655e+004,   4.21535895e-309]])

In [8]: w.wcs_world2pix(np.random.random((3,2)),1)
Out[8]: 
array([[ 615299.08679848,   18368.77731511],
       [ 540847.24016291,   13420.29274022],
       [ 719622.74011492,   20603.13017229]])
```

should the first one not fail?

cc @mdboom