always transcode the file to utf-8

people may be passing filenames to the constructor that are not utf-8,
but they will assome that calling `original_filename` returns utf-8
(because that's what it used to do).

actionpack/lib/action_dispatch/http/upload.rb

@@ -28,6 +28,7 @@ def initialize(hash) # :nodoc:
         raise(ArgumentError, ':tempfile is required') unless @tempfile
 
         @original_filename = hash[:filename]
+        @original_filename &&= @original_filename.encode "UTF-8"
         @content_type      = hash[:type]
         @headers           = hash[:head]
       end

actionpack/test/dispatch/uploaded_file_test.rb

@@ -18,6 +18,12 @@ def test_filename_should_be_in_utf_8
       assert_equal "UTF-8", uf.original_filename.encoding.to_s
     end
 
+    def test_filename_should_always_be_in_utf_8
+      uf = Http::UploadedFile.new(:filename => 'foo'.encode(Encoding::SHIFT_JIS),
+                                  :tempfile => Object.new)
+      assert_equal "UTF-8", uf.original_filename.encoding.to_s
+    end
+
     def test_content_type
       uf = Http::UploadedFile.new(:type => 'foo', :tempfile => Object.new)
       assert_equal 'foo', uf.content_type