Allow 1 mode images to be inverted

Author: radarhere

Tests/test_imageops.py

@@ -63,6 +63,7 @@ def test_sanity():
     ImageOps.grayscale(hopper("L"))
     ImageOps.grayscale(hopper("RGB"))
 
+    ImageOps.invert(hopper("1"))
     ImageOps.invert(hopper("L"))
     ImageOps.invert(hopper("RGB"))
 

src/PIL/ImageOps.py

@@ -523,7 +523,7 @@ def invert(image):
     lut = []
     for i in range(256):
         lut.append(255 - i)
-    return _lut(image, lut)
+    return image.point(lut) if image.mode == "1" else _lut(image, lut)
 
 
 def mirror(image):