typo fix

incorrect array access

lib/persisted-model.js

@@ -908,7 +908,7 @@ module.exports = function(registry) {
       // set options.explicitSinceSource to true in model config
       // to explicitly set argument source and ensure its passed
       if (options.explicitSinceSource) {
-        diffOptions[0].accepts.http = {source: 'query'};
+        diffOptions.accepts[0].http = {source: 'query'};
       }
       setRemoting(PersistedModel, 'diff', diffOptions);