Change order of arguments for `wcs.compare`.

In light of #2503, which adds a feature to make WCS comparisons "fuzzy", it became obvious that the API for `wcs.compare` had the arguments in the wrong order (the flags should be an optional kwarg, not the first argument).  This *only* changes the argument order, so hopefully is amenable to squeeze into 0.4.0 and we won't have to do a deprecation cycle for when #2503 is ultimately merged post-0.4.