Ensure image is mutable before saving

Tests/test_image.py

@@ -3,6 +3,8 @@
 from PIL import Image
 from PIL._util import py3
 import os
+import sys
+import shutil
 
 
 class TestImage(PillowTestCase):
@@ -121,6 +123,16 @@ def test_internals(self):
         im.paste(0, (0, 0, 100, 100))
         self.assertFalse(im.readonly)
 
+    @unittest.skipIf(sys.platform.startswith('win32'),
+                     "Test requires opening tempfile twice")
+    def test_readonly_save(self):
+        temp_file = self.tempfile("temp.bmp")
+        shutil.copy("Tests/images/rgb32bf-rgba.bmp", temp_file)
+
+        im = Image.open(temp_file)
+        self.assertTrue(im.readonly)
+        im.save(temp_file)
+
     def test_dump(self):
         im = Image.new("L", (10, 10))
         im._dump(self.tempfile("temp_L.ppm"))

src/PIL/Image.py

@@ -1958,7 +1958,7 @@ def save(self, fp, format=None, **params):
             filename = fp.name
 
         # may mutate self!
-        self.load()
+        self._ensure_mutable()
 
         save_all = params.pop('save_all', False)
         self.encoderinfo = params