Actually remember last valid container (jverkoey#724)

In my jverkoey#723 PR I forgot to add code to actually remember the last valid container when the flag is set.

src/attributedlabel/src/NIAttributedLabel.m

@@ -216,6 +216,9 @@ - (void)setRememberLastValidContainer:(BOOL)rememberLastValidContainer {
 }
 
 - (void)setAccessibilityContainer:(id)accessibilityContainer {
+  if (self.isContainerDataValid && self.rememberLastValidContainer) {
+    self.lastValidContainer = self.accessibilityContainer;
+  }
   self.isContainerDataValid = NO;
   [super setAccessibilityContainer:accessibilityContainer];
 }