Bugfix for when file is not present on cloud

Currently paperclip fails with `NoMethodError` on `nil` when file is not
present. Since `nil` is proper response from fog on missing files, and
this method already returns `false` if any error happen (see line 178),
returning `false` here would be properly handled on client code.

[closes #2173]

lib/paperclip/storage/fog.rb

@@ -170,6 +170,7 @@ module Paperclip
         log("copying #{path(style)} to local file #{local_dest_path}")
         ::File.open(local_dest_path, 'wb') do |local_file|
           file = directory.files.get(path(style))
+          return false unless file
           local_file.write(file.body)
         end
       rescue ::Fog::Errors::Error => e

spec/paperclip/storage/fog_spec.rb

@@ -183,6 +183,13 @@ describe Paperclip::Storage::Fog do
         tempfile.close
       end
 
+      it 'is able to be handled when missing while copying to a local file' do
+        tempfile = Tempfile.new("known_location")
+        tempfile.binmode
+        assert_equal false, @dummy.avatar.copy_to_local_file(:original, tempfile.path)
+        tempfile.close
+      end
+
       it "passes the content type to the Fog::Storage::AWS::Files instance" do
         Fog::Storage::AWS::Files.any_instance.expects(:create).with do |hash|
           hash[:content_type]