Changed encoderinfo to have priority over info when saving GIF images

Tests/test_file_gif.py

@@ -305,9 +305,12 @@ class TestFileGif(PillowTestCase):
 
         out = self.tempfile('temp.gif')
         im = Image.new('L', (100, 100), '#000')
-        im.save(out, duration=duration)
-        reread = Image.open(out)
 
+        # Check that the argument has priority over the info settings
+        im.info['duration'] = 100
+        im.save(out, duration=duration)
+
+        reread = Image.open(out)
         self.assertEqual(reread.info['duration'], duration)
 
     def test_multiple_duration(self):

src/PIL/GifImagePlugin.py

@@ -457,7 +457,8 @@ def _save_all(im, fp, filename):
 
 
 def _save(im, fp, filename, save_all=False):
-    im.encoderinfo.update(im.info)
+    for k, v in im.info.items():
+        im.encoderinfo.setdefault(k, v)
     # header
     try:
         palette = im.encoderinfo["palette"]