Disable the CSRF warning in WebRequestsController

I see this warning every time WebRequestsController processes a POST request.

```
I, [2014-10-10T12:23:45.629681 #31902]  INFO -- : [1c31109c-237e-4db8-b5c2-f63832b4864f] Started POST "/users/1/web_requests/27/***" for ***.***.***.*** at 2014-10-10 12:23:45 +0900
I, [2014-10-10T12:23:45.631097 #31902]  INFO -- : [1c31109c-237e-4db8-b5c2-f63832b4864f] Processing by WebRequestsController#handle_request as */*
I, [2014-10-10T12:23:45.631215 #31902]  INFO -- : [1c31109c-237e-4db8-b5c2-f63832b4864f]   Parameters: {...}, "user_id"=>"1", "agent_id"=>"27", "secret"=>"***", "web_request"=>{...}}}
W, [2014-10-10T12:23:45.631523 #31902]  WARN -- : [1c31109c-237e-4db8-b5c2-f63832b4864f] Can't verify CSRF token authenticity
I, [2014-10-10T12:23:45.663737 #31902]  INFO -- : [1c31109c-237e-4db8-b5c2-f63832b4864f]   Rendered text template (0.0ms)
I, [2014-10-10T12:23:45.663929 #31902]  INFO -- : [1c31109c-237e-4db8-b5c2-f63832b4864f] Completed 201 Created in 33ms (Views: 0.4ms | ActiveRecord: 15.0ms)
```

Seems the warning is just a warning and harmless, but I'd rather not see it all the time.