Fix #exists? returning true after Fog file deletion

Fixes #2387, Refs. bffbaaf

lib/carrierwave/storage/fog.rb

@@ -242,7 +242,9 @@ def content_type=(new_content_type)
         #
         def delete
           # avoid a get by just using local reference
-          directory.files.new(:key => path).destroy
+          directory.files.new(:key => path).destroy.tap do |result|
+            @file = nil if result
+          end
         end
 
         ##

spec/storage/fog_helper.rb

@@ -258,6 +258,10 @@ class FogSpec#{fog_credentials[:provider]}Uploader < CarrierWave::Uploader::Base
             it "should not return false for content type" do
               expect(@fog_file.content_type).not_to be false
             end
+
+            it "should let #exists? be false" do
+              expect(@fog_file.exists?).to be false
+            end
           end
         end