Bye request should use POST not GET

AppRTCDemo is failing to cleanly exit a room because it sends a GET request to /bye. The request to /bye should be a POST request. Because the /bye request is failing, the room is still marked as "full" and rejoining will fail.

BUG=
R=tkchin@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/47759004

Patch from Chuck Hays <haysc@webrtc.org>.

Cr-Commit-Position: refs/heads/master@{#8860}
diff --git a/talk/examples/objc/AppRTCDemo/ARDAppEngineClient.m b/talk/examples/objc/AppRTCDemo/ARDAppEngineClient.m
index 0f3ad0a..760a171 100644
--- a/talk/examples/objc/AppRTCDemo/ARDAppEngineClient.m
+++ b/talk/examples/objc/AppRTCDemo/ARDAppEngineClient.m
@@ -141,7 +141,8 @@
   NSString *urlString =
       [NSString stringWithFormat:kARDRoomServerLeaveFormat, roomId, clientId];
   NSURL *url = [NSURL URLWithString:urlString];
-  NSURLRequest *request = [NSURLRequest requestWithURL:url];
+  NSMutableURLRequest *request = [NSMutableURLRequest requestWithURL:url];
+  request.HTTPMethod = @"POST";
   NSURLResponse *response = nil;
   NSError *error = nil;
   // We want a synchronous request so that we know that we've left the room on