Update onBeforeSend d.ts and docs (#566)

Author: AlekseyMartynov

docs/client-side-with-jquery.md

@@ -90,7 +90,7 @@ When you call the `DevExpress.data.AspNet.createStore` method, pass an object wi
 - `onBeforeSend` - a function that customizes the request before it is sent.
 
     ```js
-    onBeforeSend: (operation, ajaxSettings) => void
+    onBeforeSend: (operation, ajaxSettings) => void | PromiseLike
     ```
 
     Parameter  | Type | Description

js-test/check-ts-compilation.ts

@@ -38,3 +38,11 @@ AspNetData.createStore({
 
     onPush: (changes) => console.log(changes.length)
 })
+
+AspNetData.createStore({
+    onBeforeSend: () => "any" // backward compatibility
+});
+
+AspNetData.createStore({
+    onBeforeSend: Promise.resolve
+});

js/dx.aspnet.data.d.ts

@@ -20,7 +20,7 @@ interface Options {
     loadMode?: "processed" | "raw",
     cacheRawData?: boolean,
 
-    onBeforeSend?: (operation: string, ajaxSettings: JQueryAjaxSettings) => void,
+    onBeforeSend?: (operation: string, ajaxSettings: JQueryAjaxSettings) => void | PromiseLike<any>| any,
     onAjaxError?: (e: { xhr: JQueryXHR, error: string | Error }) => void
 
     onLoading?: (loadOptions: any) => void;