Restore UploadedFile compatibility with IO.copy_stream

In #28676 the `#to_path` method was
added to `ActionDispatch::Http::UploadedFile`. This broke usage with
`IO.copy_stream`:

  source = ActionDispatch::Http::UploadedFile.new(...)
  IO.copy_stream(source, destination)
  # ~> TypeError: can't convert ActionDispatch::Http::UploadedFile to IO (ActionDispatch::Http::UploadedFile#to_io gives Tempfile)

Normally `IO.copy_stream` just calls `#read` on the source object.
However, when `#to_path` is defined, `IO.copy_stream` calls `#to_io` in
order to retrieve the raw `File` object. In that case it trips up,
because `ActionDispatch::Http::UploadedFile#to_io` returned a `Tempfile`
object, which is not an `IO` subclass.

We fix this by having `#to_io` return an actual `File` object.

actionpack/lib/action_dispatch/http/upload.rb

@@ -20,7 +20,6 @@ class UploadedFile
       # A +Tempfile+ object with the actual uploaded file. Note that some of
       # its interface is available directly.
       attr_accessor :tempfile
-      alias :to_io :tempfile
 
       # A string with the headers of the multipart request.
       attr_accessor :headers
@@ -84,6 +83,10 @@ def size
       def eof?
         @tempfile.eof?
       end
+
+      def to_io
+        @tempfile.to_io
+      end
     end
   end
 end

actionpack/test/dispatch/uploaded_file_test.rb

@@ -2,6 +2,7 @@
 
 require "abstract_unit"
 require "tempfile"
+require "stringio"
 
 module ActionDispatch
   class UploadedFileTest < ActiveSupport::TestCase
@@ -49,10 +50,10 @@ def test_tempfile
       assert_equal tf, uf.tempfile
     end
 
-    def test_to_io_returns_tempfile
+    def test_to_io_returns_file
       tf = Tempfile.new
       uf = Http::UploadedFile.new(tempfile: tf)
-      assert_equal tf, uf.to_io
+      assert_equal tf.to_io, uf.to_io
     end
 
     def test_delegates_path_to_tempfile
@@ -115,5 +116,15 @@ def test_delegate_to_path_to_tempfile
       uf = Http::UploadedFile.new(tempfile: tf)
       assert_equal tf.to_path, uf.to_path
     end
+
+    def test_io_copy_stream
+      tf = Tempfile.new
+      tf << "thunderhorse"
+      tf.rewind
+      uf = Http::UploadedFile.new(tempfile: tf)
+      result = StringIO.new
+      IO.copy_stream(uf, result)
+      assert_equal "thunderhorse", result.string
+    end
   end
 end