Fix zadd when key already exists with same score

Real redis treats it as a no-op rather than replacing it. This means
that if the score was -0.0 and is being changed to +0.0 (or vice versa),
it isn't updated. It also affects watches.

fakeredis/_server.py

@@ -1891,8 +1891,9 @@ def zadd(self, key, *args):
             items.append((score, args[i + 1]))
         old_len = len(key.value)
         for item in items:
-            key.value[item[1]] = item[0]
-            key.updated()
+            if item[1] not in key.value or key.value[item[1]] != item[0]:
+                key.value[item[1]] = item[0]
+                key.updated()
         return len(key.value) - old_len
 
     @command((Key(ZSet),))

test_fakeredis.py

@@ -1765,6 +1765,12 @@ def test_zadd_empty(self):
         with self.assertRaises(redis.RedisError):
             self.zadd('foo', {})
 
+    def test_zadd_minus_zero(self):
+        # Changing -0 to +0 is ignored
+        self.zadd('foo', {'a': -0.0})
+        self.zadd('foo', {'a': 0.0})
+        self.assertEqual(self.redis.execute_command('zscore', 'foo', 'a'), b'-0')
+
     def test_zadd_wrong_type(self):
         self.redis.sadd('foo', 'bar')
         with self.assertRaises(redis.ResponseError):