Added documentation for hls.xhr.beforeRequest with examples

@@ -19,6 +19,7 @@ Play back HLS with video.js, even where it's not natively supported.
     - [hls.bandwidth](#hlsbandwidth)
     - [hls.bytesReceived](#hlsbytesreceived)
     - [hls.selectPlaylist](#hlsselectplaylist)
+    - [hls.xhr](#hlsxhr)
   - [Events](#events)
     - [loadedmetadata](#loadedmetadata)
     - [loadedplaylist](#loadedplaylist)
@@ -194,6 +195,44 @@ segment is downloaded. You can override this function to provide your
 adaptive streaming logic. You must, however, be sure to return a valid
 media playlist object that is present in `player.hls.master`.
 
+#### hls.xhr
+Type: `function`
+
+The xhr function that is used by HLS internally is exposed on the per-
+player `hls` object. While it is possible, we do not recommend replacing
+the function with your own implementation. Instead, the `xhr` provides
+the ability to specify a `beforeRequest` function that will be called
+with an object containing the options that will be used to create the
+xhr request.
+
+Example:
+```javascript
+player.hls.xhr.beforeRequest = function(options) {
+  options.uri = options.uri.replace('example.com', 'foo.com');
+
+  return options;
+};
+```
+
+The global `videojs.Hls` also exposes an `xhr` property. Specifying a
+`beforeRequest` function on that will allow you to intercept the options
+for *all* requests in every player on a page.
+
+Example
+```javascript
+videojs.Hls.xhr.beforeRequest = function(options) {
+  /*
+   * Modifications to requests that will affect every player.
+   */
+
+  return options;
+};
+```
+
+For information on the type of options that you can modify see the
+documentation at [https://github.com/Raynos/xhr](https://github.com/Raynos/xhr).
+
+
 ### Events
 Standard HTML video events are handled by video.js automatically and
 are triggered on the player object. In addition, there are a couple

@@ -3366,17 +3366,15 @@ QUnit.test('selectPlaylist does not fail if getComputedStyle returns null', func
 QUnit.test('Allows specifying the beforeRequest functionon the player', function() {
   let beforeRequestCalled = false;
 
-  this.player.ready(function() {
-    this.hls.xhr.beforeRequest = function() {
-      beforeRequestCalled = true;
-    };
-  });
   this.player.src({
     src: 'master.m3u8',
     type: 'application/vnd.apple.mpegurl'
   });
-
   openMediaSource(this.player, this.clock);
+
+  this.player.hls.xhr.beforeRequest = function() {
+    beforeRequestCalled = true;
+  };
   // master
   standardXHRResponse(this.requests.shift());
   // media
@@ -3396,6 +3394,9 @@ QUnit.test('Allows specifying the beforeRequest function globally', function() {
     src: 'master.m3u8',
     type: 'application/vnd.apple.mpegurl'
   });
+  openMediaSource(this.player, this.clock);
+  // master
+  standardXHRResponse(this.requests.shift());
 
   QUnit.ok(beforeRequestCalled, 'beforeRequest was called');
 
@@ -3414,13 +3415,11 @@ QUnit.test('Allows overriding the global beforeRequest function', function() {
     src: 'master.m3u8',
     type: 'application/vnd.apple.mpegurl'
   });
-  this.player.ready(function() {
-    this.hls.xhr.beforeRequest = function() {
-      beforeLocalRequestCalled++;
-    };
-  });
-
   openMediaSource(this.player, this.clock);
+
+  this.player.hls.xhr.beforeRequest = function() {
+    beforeLocalRequestCalled++;
+  };
   // master
   standardXHRResponse(this.requests.shift());
   // media