Refs #1629. Fixed: aSet removeObject: for an object not a member is not an error.

Foundation/CPSet/_CPConcreteMutableSet.j

@@ -108,13 +108,19 @@ var hasOwnProperty = Object.prototype.hasOwnProperty;
 */
 - (void)removeObject:(id)anObject
 {
-    var object = [self member:anObject];
-
-    if (object === nil || object === undefined)
+    // Removing nil is an error.
+    if (anObject === nil || anObject === undefined)
         [CPException raise:CPInvalidArgumentException reason:@"attempt to remove nil or undefined"];
 
-    delete _contents[[object UID]];
-    _count--;
+    // anObject might be isEqual: another object in the set. We need the exact instance so we can remove it by UID.
+    var object = [self member:anObject];
+
+    // ...but removing an object not present in the set is not an error.
+    if (object !== nil)
+    {
+        delete _contents[[object UID]];
+        _count--;
+    }
 }
 
 /*

Tests/Foundation/CPSetTest.j

@@ -182,16 +182,20 @@
     [set2 addObject:dict1];
     [set2 removeObject:dict2];
     [self assertTrue:[set2 count] === 0];
+
+    // Removing an object not in the set is not an error.
+    [set2 removeObject:dict2];
 }
 
 - (void)testRemoveZeroObject
 {
     var set = [CPSet new];
 
+    // In Objective-J this is equivalent to [set addObject:[CPNumber numberWithInt:0]];
     [set addObject:0];
-    // [self assertTrue:[set containsObject:0]];
+    [self assertTrue:[set containsObject:0] message:@"adding 0 to a set should work"];
     [set removeObject:0];
-    [self assertFalse:[set containsObject:0]];
+    [self assertFalse:[set containsObject:0] message:@"removing 0 from a set should work"];
 }
 
 - (void)testAddNilObject