fix undefined method 'variant' for nil:NilClass ActionController #render_to_text change API

I've tried to use xray-rails with rails 4.1 and it just blows up with ```nil``` no method error. It looks like we have to provide request object for ActionController::Base directly.

Here is the patch to fix it 

```
NoMethodError - undefined method `variant' for nil:NilClass:
  actionpack (4.1.0.rc1) lib/abstract_controller/rendering.rb:109:in `_normalize_render'
  actionpack (4.1.0.rc1) lib/abstract_controller/rendering.rb:42:in `render_to_string'
  actionpack (4.1.0.rc1) lib/action_controller/metal/rendering.rb:21:in `render_to_string'
  xray-rails (0.1.12) lib/xray/middleware.rb:65:in `xray_bar'
  xray-rails (0.1.12) lib/xray/middleware.rb:40:in `block in call'
  xray-rails (0.1.12) lib/xray/middleware.rb:40:in `call'
  warden (1.2.3) lib/warden/manager.rb:35:in `block in call'
  warden (1.2.3) lib/warden/manager.rb:34:in `call'
  rack (1.5.2) lib/rack/etag.rb:23:in `call'
  rack (1.5.2) lib/rack/conditionalget.rb:25:in `call'
  rack (1.5.2) lib/rack/head.rb:11:in `call' 
```