Use the ID of the default auth source, not "default"

This makes it possible for existing deployments to have this working with SSO. Otherwise, the user would end up authenticated with the "default" auth source when accessing the web interface, and "another" auth source when logging in via SAML.

modules/core/lib/Controller/Login.php

@@ -144,8 +144,18 @@ class Login
                 $t->data['sources'] = $this->sources;
                 return $t;
             }
+
             // we have a default, use that one
             $as = 'default';
+            foreach ($this->sources as $id => $source) {
+                if ($id === 'default') {
+                    continue;
+                }
+                if ($source === $this->sources['default']) {
+                    $as = $id;
+                    break;
+                }
+            }
         }
 
         // auth source defined, check if valid