Prevent double-validation in LoginSerializer

Modified branching statement to prevent double-validation when using "email" as the authentication method with Allauth.

Author: ducheneaut

rest_auth/serializers.py

@@ -71,7 +71,7 @@ def validate(self, attrs):
                 user = self._validate_email(email, password)
 
             # Authentication through username
-            if app_settings.AUTHENTICATION_METHOD == app_settings.AuthenticationMethod.USERNAME:
+            elif app_settings.AUTHENTICATION_METHOD == app_settings.AuthenticationMethod.USERNAME:
                 user = self._validate_username(username, password)
 
             # Authentication through either username or email