Merge pull request cocoabits#34 from swt2c/fix_double_viewWillAppear

Avoid calling viewWillAppear twice on OSX 10.10

MASPreferencesWindowController.m

@@ -303,8 +303,12 @@ - (void)setSelectedViewController:(NSViewController <MASPreferencesViewControlle
 #else
     _selectedViewController = [controller retain];
 #endif
-    if ([controller respondsToSelector:@selector(viewWillAppear)])
-        [controller viewWillAppear];
+    // In OSX 10.10, setContentView below calls viewWillAppear.  We still want to call viewWillAppear on < 10.10,
+    // so the check below avoids calling viewWillAppear twice on 10.10.
+    // See https://github.com/shpakovski/MASPreferences/issues/32 for more info.
+    if (![NSViewController instancesRespondToSelector:@selector(viewWillAppear)])
+        if ([controller respondsToSelector:@selector(viewWillAppear)])
+            [controller viewWillAppear];
 
     [self.window setContentView:controllerView];
     [self.window recalculateKeyViewLoop];