[Enhancement] Added GroupHeaderStyle property for iOS specific ListView class (#4616)

* Added group header style property for ios specific list view class

* removed redundant test line

Xamarin.Forms.Core/PlatformConfiguration/iOSSpecific/GroupHeaderStyle.cs

@@ -0,0 +1,9 @@
+
+namespace Xamarin.Forms.PlatformConfiguration.iOSSpecific
+{
+	public enum GroupHeaderStyle
+	{
+		Plain,
+		Grouped
+	}
+}

Xamarin.Forms.Core/PlatformConfiguration/iOSSpecific/ListView.cs

@@ -26,5 +26,28 @@
 			SetSeparatorStyle(config.Element, value);
 			return config;
 		}
+
+		public static readonly BindableProperty GroupHeaderStyleProperty = BindableProperty.Create(nameof(GroupHeaderStyle), typeof(GroupHeaderStyle), typeof(FormsElement), GroupHeaderStyle.Plain);
+
+		public static GroupHeaderStyle GetGroupHeaderStyle(BindableObject element)
+		{
+			return (GroupHeaderStyle)element.GetValue(GroupHeaderStyleProperty);
+		}
+
+		public static void SetGroupHeaderStyle(BindableObject element, GroupHeaderStyle value)
+		{
+			element.SetValue(GroupHeaderStyleProperty, value);
+		}
+
+		public static GroupHeaderStyle GetGroupHeaderStyle(this IPlatformElementConfiguration<iOS, FormsElement> config)
+		{
+			return GetGroupHeaderStyle(config.Element);
+		}
+
+		public static IPlatformElementConfiguration<iOS, FormsElement> SetGroupHeaderStyle(this IPlatformElementConfiguration<iOS, FormsElement> config, GroupHeaderStyle value)
+		{
+			SetGroupHeaderStyle(config.Element, value);
+			return config;
+		}
 	}
 }

Xamarin.Forms.Platform.iOS/Renderers/ListViewRenderer.cs

@@ -1368,7 +1368,10 @@ namespace Xamarin.Forms.Platform.iOS
 		internal bool _usingLargeTitles;
 		bool _isRefreshing;
 
-		public FormsUITableViewController(ListView element, bool usingLargeTitles)
+		public FormsUITableViewController(ListView element, bool usingLargeTitles) 
+		: base(element.OnThisPlatform().GetGroupHeaderStyle() == GroupHeaderStyle.Plain 
+			? UITableViewStyle.Plain 
+			  : UITableViewStyle.Grouped)
 		{
 			if (Forms.IsiOS9OrNewer)
 				TableView.CellLayoutMarginsFollowReadableWidth = false;