Allow #original_filename= to receive nil argument

lib/paperclip/io_adapters/abstract_adapter.rb

@@ -20,6 +20,7 @@ module Paperclip
     end
 
     def original_filename=(new_filename)
+      return unless new_filename
       @original_filename = new_filename.gsub(OS_RESTRICTED_CHARACTERS, "_")
     end
 

spec/paperclip/io_adapters/abstract_adapter_spec.rb

@@ -67,4 +67,11 @@ describe Paperclip::AbstractAdapter do
     @adapter.original_filename = "file.png"
     expect(@adapter.send(:destination).path).to end_with(".png")
   end
+
+  context "#original_filename=" do
+    it "should not fail with a nil original filename" do
+      adapter = TestAdapter.new
+      expect{ adapter.original_filename = nil }.not_to raise_error
+    end
+  end
 end