Retrieve omniauth_identity automatically when accessed for first ti…

…me (#29)

I often forget to call `retrieve_omniauth_identity` before trying to
access the `omniauth_identity` attribute in
`before_omniauth_callback_route`.

I think we can make it automatically retrieve `omniauth_identity` the
first time it's called.

I haven't removed the `retrieve_omniauth_identity` method since it might
already be called by users.

lib/rodauth/features/omniauth.rb

@@ -58,14 +58,12 @@ def handle_omniauth_callback
     def _handle_omniauth_callback
       before_omniauth_callback_route
 
-      retrieve_omniauth_identity
-
-      if !account && omniauth_identity
-        account_from_omniauth_identity
-      end
-
       unless account
-        account_from_omniauth
+        if omniauth_identity
+          account_from_omniauth_identity
+        else
+          account_from_omniauth
+        end
       end
 
       if account && !open_account?
@@ -100,8 +98,12 @@ def _handle_omniauth_callback
       end
     end
 
+    def omniauth_identity
+      @omniauth_identity ||= retrieve_omniauth_identity
+    end
+
     def retrieve_omniauth_identity
-      @omniauth_identity = _retrieve_omniauth_identity(omniauth_provider, omniauth_uid)
+      _retrieve_omniauth_identity(omniauth_provider, omniauth_uid)
     end
 
     def account_from_omniauth_identity
@@ -142,8 +144,6 @@ def after_close_account
       remove_omniauth_identities
     end
 
-    attr_reader :omniauth_identity
-
     def omniauth_second_factor?
       features.include?(:two_factor_base) && uses_two_factor_authentication? && omniauth_two_factors?
     end