[iOS] allow mailto: uri (#2646)

* [iOS] allow mailto: uri

same as #1311 but for mailto:

 - fixes #2628

* Add mailto test

Xamarin.Forms.Controls.Issues/Xamarin.Forms.Controls.Issues.Shared/Bugzilla60691.cs

@@ -18,7 +18,8 @@ namespace Xamarin.Forms.Controls.Issues
 			Content = new StackLayout
 			{
 				Children = {
-					new Button { Text = "Call 123 4567", AutomationId = "tel", Command = new Command(() => Device.OpenUri(new System.Uri("tel:123 4567"))) }
+					new Button { Text = "Call 123 4567", AutomationId = "tel", Command = new Command(() => Device.OpenUri(new System.Uri("tel:123 4567"))) },
+					new Button { Text = "Mail support@xamarin.com", AutomationId = "mailto", Command = new Command(() => Device.OpenUri(new System.Uri("mailto:support@xamarin.com"))) }
 				}
 			};
 		}

Xamarin.Forms.Platform.iOS/Forms.cs

@@ -260,7 +260,7 @@ namespace Xamarin.Forms
 			{
 				NSUrl url;
 
-				if (uri.Scheme == "tel")
+				if (uri.Scheme == "tel" || uri.Scheme == "mailto")
 					url = new NSUrl(uri.AbsoluteUri);
 				else
 					url = NSUrl.FromString(uri.OriginalString) ?? new NSUrl(uri.Scheme, uri.Host, uri.PathAndQuery);