[iOS] Correctly size subtitle label before calculating title view width (#928)

ios/Helpers/RCCTitleViewHelper.m

@@ -156,7 +156,8 @@ navigationController:(UINavigationController*)navigationController
     CGRect labelframe = subtitleLabel.frame;
     labelframe.size = labelSize;
     subtitleLabel.frame = labelframe;
-    
+    [subtitleLabel sizeToFit];
+
     [self.titleView addSubview:subtitleLabel];
     
     return subtitleLabel;