Do not modify supplied Exif object

Author: radarhere

Tests/test_file_jpeg.py

@@ -207,10 +207,10 @@ def test_dpi_jfif_cm(self) -> None:
     def test_dpi_with_keep_rgb(self, tmp_path: Path) -> None:
         im = Image.new("RGB", (20, 20), (255, 0, 0))
         f = tmp_path / "temp.jpg"
-        im.save(f, "JPEG", dpi=(72, 72), keep_rgb=True)
+        im.save(f, "JPEG", dpi=(96, 96), keep_rgb=True)
 
         with Image.open(f) as reloaded:
-            assert reloaded.info["dpi"] == (72, 72)
+            assert reloaded.info["dpi"] == (96, 96)
             assert_image_equal(reloaded, im)
 
     def test_icc(self, tmp_path: Path) -> None:

src/PIL/JpegImagePlugin.py

@@ -818,10 +818,10 @@ def validate_qtables(
     if "dpi" in info:
         dpi = tuple(round(x) for x in info["dpi"])
         if keep_rgb:
-            if not isinstance(exif, Image.Exif):
-                exif_object = Image.Exif()
-                exif_object.load(exif)
-                exif = exif_object
+            exif_object = Image.Exif()
+            exif_object.load(exif.tobytes() if isinstance(exif, Image.Exif) else exif)
+            exif = exif_object
+
             exif[0x011A] = dpi
         else:
             jfif_dpi = dpi