Fix issue where you couldn't edit your canonical domain.
Fix issue where you couldn't edit your canonical domain.

This logic was checking to see if the domain passed in was the current canonical,
but using the domain you were _trying_ to change it to.
We should be checking for the existing PK to see if you're editing,
not the new domain.

rtfd/readthedocs.org,davidfischer/readthedocs.org,rtfd/readthedocs.org,davidfischer/readthedocs.org,safwanrahman/readthedocs.org,safwanrahman/readthedocs.org,davidfischer/readthedocs.org,safwanrahman/readthedocs.org,rtfd/readthedocs.org,davidfischer/readthedocs.org,rtfd/readthedocs.org,safwanrahman/readthedocs.org