Use x_auth to login after creating a new account.

df6c59f692fee545758775ca0b242118f6d42a91 broke signup which relied on using x_auth to login after creating the account. Sending the user through the web-browser authentication route doesn't make sense when we know they have a normal account so this brings back x_auth for just the register method.
Most of these specific messages haven't actually been working correctly since call_rest_method calls throwUserError for any HTTP result > 404.