Fixed: "[CPString count] unrecognized selector" error when binding an array controller's content array to a key of another AC's selection.

This change also avoids wrapping a selection proxy in a KVO array proxy, which isn't helpful and probably hurts performance.

AppKit/CPArrayController.j

@@ -1076,22 +1076,45 @@
     var destination = [_info objectForKey:CPObservedObjectKey],
         keyPath = [_info objectForKey:CPObservedKeyPathKey],
         options = [_info objectForKey:CPOptionsKey],
-        isCompound = [self handlesContentAsCompoundValue];
+        isCompound = [self handlesContentAsCompoundValue],
+        dotIndex = keyPath.lastIndexOf("."),
+        firstPart = dotIndex !== CPNotFound ? keyPath.substring(0, dotIndex) : nil,
+        isSelectionProxy = firstPart && [[destination valueForKeyPath:firstPart] isKindOfClass:CPControllerSelectionProxy];
 
-    if (!isCompound)
+    if (!isCompound && !isSelectionProxy)
     {
         newValue = [destination mutableArrayValueForKeyPath:keyPath];
     }
     else
     {
-        // handlesContentAsCompoundValue == YES so we cannot just set up a proxy.
+        // 1. If handlesContentAsCompoundValue we cannot just set up a proxy.
         // Every read and every write must go through transformValue and
         // reverseTransformValue, and the resulting object cannot be described by
         // a key path.
+
+        // 2. If isSelectionProxy, we don't want to proxy a proxy - that's bad
+        // for performance and won't work with markers.
+
         newValue = [destination valueForKeyPath:keyPath];
     }
 
-    newValue = [self transformValue:newValue withOptions:options];
+    var isPlaceholder = CPIsControllerMarker(newValue);
+    if (isPlaceholder)
+    {
+        if (newValue === CPNotApplicableMarker && [options objectForKey:CPRaisesForNotApplicableKeysBindingOption])
+        {
+           [CPException raise:CPGenericException
+                       reason:@"can't transform non applicable key on: " + _source + " value: " + newValue];
+        }
+
+        newValue = [self _placeholderForMarker:newValue];
+
+        // This seems to be what Cocoa does.
+        if (!newValue)
+            newValue = [CPMutableArray array];
+    }
+    else
+        newValue = [self transformValue:newValue withOptions:options];
 
     if (isCompound)
     {