Ensure image processing gets disabled again, even after exceptions

Otherwise this leaves it enabled if the processing failed, which then
makes other specs fail where they expect the image not being processed
(for example still have the initial set dimensions, instead of the one
read from image after processing).

spec/models/photo_spec.rb

@@ -6,9 +6,9 @@
 
 def with_carrierwave_processing(&block)
   UnprocessedImage.enable_processing = true
-  val = yield
+  yield
+ensure
   UnprocessedImage.enable_processing = false
-  val
 end
 
 describe Photo, :type => :model do