[X] Don't remove enclosing quotes (xamarin#12813) fixes xamarin#12763

Quotes are already parsed, and handled elsewhere

- fixes xamarin#12763

Xamarin.Forms.Xaml.UnitTests/Issues/Gh12763.xaml

@@ -0,0 +1,4 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<ContentPage xmlns="http://xamarin.com/schemas/2014/forms" xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml" x:Class="Xamarin.Forms.Xaml.UnitTests.Gh12763">
+    <Label x:Name="label" Text="{Binding ., StringFormat='&quot;{0}&quot;', Source=Foo}" />
+</ContentPage>

Xamarin.Forms.Xaml.UnitTests/Issues/Gh12763.xaml.cs

@@ -0,0 +1,33 @@
+// Copyright (c) Microsoft Corporation.
+// Licensed under the MIT License.
+using System;
+using System.Collections.Generic;
+using NUnit.Framework;
+using Xamarin.Forms;
+using Xamarin.Forms.Core.UnitTests;
+
+namespace Xamarin.Forms.Xaml.UnitTests
+{
+	public partial class Gh12763 : ContentPage
+	{
+		public Gh12763() => InitializeComponent();
+		public Gh12763(bool useCompiledXaml)
+		{
+			//this stub will be replaced at compile time
+		}
+
+		[TestFixture]
+		class Tests
+		{
+			[SetUp] public void Setup() => Device.PlatformServices = new MockPlatformServices();
+			[TearDown] public void TearDown() => Device.PlatformServices = null;
+
+			[Test]
+			public void QuotesInStringFormat([Values(false, true)] bool useCompiledXaml)
+			{
+				var layout = new Gh12763(useCompiledXaml);
+				Assert.That(layout.label.Text, Is.EqualTo("\"Foo\""));
+			}
+		}
+	}
+}

Xamarin.Forms.Xaml/MarkupExpressionParser.cs

@@ -224,17 +224,6 @@ string GetNextPiece(IServiceProvider serviceProvider, ref string remaining, out
 			while (piece.Length > 0 && char.IsWhiteSpace(piece[piece.Length - 1]))
 				piece.Length--;
 
-			if (piece.Length >= 2)
-			{
-				char first = piece[0];
-				char last = piece[piece.Length - 1];
-				if ((first == '\'' && last == '\'') || (first == '"' && last == '"'))
-				{
-					piece.Remove(piece.Length - 1, 1);
-					piece.Remove(0, 1);
-				}
-			}
-
 			return piece.ToString();
 		}