fix(loading): include cssClass in the Loading options

added to test as well

fixes #6365

ionic/components/loading/loading.ts

@@ -224,6 +224,7 @@ class LoadingCmp {
 export interface LoadingOptions {
   spinner?: string;
   content?: string;
+  cssClass?: string;
   showBackdrop?: boolean;
   dismissOnPageChange?: boolean;
   delay?: number;