[#2523] Fix infinite-loop when remove attribute and create the same attribute again

Motivation:
The current DefaultAttributeMap cause an infinite-loop when the user removes an attribute and create the same attribute again. This regression was introduced by c3bd7a8ff1.

Modification:
Correctly break out loop

Result:
No infinite-loop anymore.

common/src/main/java/io/netty/util/DefaultAttributeMap.java

@@ -90,6 +90,9 @@ public class DefaultAttributeMap implements AttributeMap {
                     DefaultAttribute<T> attr = new DefaultAttribute<T>(head, key);
                     curr.next =  attr;
                     attr.prev = curr;
+                    return attr;
+                } else {
+                    curr = next;
                 }
             }
         }

common/src/test/java/io/netty/util/DefaultAttributeMapTest.java

@@ -67,4 +67,16 @@ public class DefaultAttributeMapTest {
         one.remove();
         assertNull(one.get());
     }
+
+    // See https://github.com/netty/netty/issues/2523
+    @Test
+    public void testSetRemove() {
+        AttributeKey<Integer> key = AttributeKey.valueOf("key");
+
+        map.attr(key).set(1);
+        assertSame(1, map.attr(key).getAndRemove());
+
+        map.attr(key).set(2);
+        assertSame(2, map.attr(key).get());
+    }
 }