Fix values normalization in feature squeezing (close #16)

(cherry picked from commit 857ab12)

Author: Irina Nicolae

art/defences/feature_squeezing.py

@@ -57,7 +57,7 @@ def __call__(self, x, y=None, bit_depth=None, clip_values=(0, 1)):
 
         x_ = x - clip_values[0]
         if clip_values[1] != 0:
-            x_ = x_ / clip_values[1]
+            x_ = x_ / (clip_values[1] - clip_values[0])
 
         max_value = np.rint(2 ** self.bit_depth - 1)
         res = (np.rint(x_ * max_value) / max_value) * (clip_values[1] - clip_values[0]) + clip_values[0]