Added I;16 PNG save

Tests/test_file_png.py

@@ -88,20 +88,13 @@ class TestFilePng(PillowTestCase):
         self.assertEqual(im.format, "PNG")
         self.assertEqual(im.get_format_mimetype(), 'image/png')
 
-        hopper("1").save(test_file)
-        Image.open(test_file)
-
-        hopper("L").save(test_file)
-        Image.open(test_file)
-
-        hopper("P").save(test_file)
-        Image.open(test_file)
-
-        hopper("RGB").save(test_file)
-        Image.open(test_file)
-
-        hopper("I").save(test_file)
-        Image.open(test_file)
+        for mode in ["1", "L", "P", "RGB", "I", "I;16"]:
+            im = hopper(mode)
+            im.save(test_file)
+            reloaded = Image.open(test_file)
+            if mode == "I;16":
+                reloaded = reloaded.convert(mode)
+            self.assert_image_equal(reloaded, im)
 
     def test_invalid_file(self):
         invalid_file = "Tests/images/flower.jpg"

src/PIL/PngImagePlugin.py

@@ -707,6 +707,7 @@ _OUTMODES = {
     "L":    ("L",       b'\x08\x00'),
     "LA":   ("LA",      b'\x08\x04'),
     "I":    ("I;16B",   b'\x10\x00'),
+    "I;16": ("I;16B",   b'\x10\x00'),
     "P;1":  ("P;1",     b'\x01\x03'),
     "P;2":  ("P;2",     b'\x02\x03'),
     "P;4":  ("P;4",     b'\x04\x03'),

src/libImaging/Pack.c

@@ -639,6 +639,7 @@ static struct {
 
     /* storage modes */
     {"I;16",    "I;16",         16,     copy2},
+    {"I;16",    "I;16B",        16,     packI16N_I16B},
     {"I;16B",   "I;16B",        16,     copy2},
     {"I;16L",   "I;16L",        16,     copy2},
     {"I;16",    "I;16N",        16,     packI16N_I16}, // LibTiff native->image endian.