[msbuild] Change MtouchHttpClientHandler default (#4172)

If the Http Client value isn't set in the csproj, we should default to `NSUrlSessionHandler` which is also what the Xamarin.iOS Analysis rules try to enforce.

msbuild/Xamarin.iOS.Tasks.Core/Xamarin.iOS.Common.props

@@ -40,7 +40,7 @@ Copyright (C) 2013-2016 Xamarin. All rights reserved.
 		<MtouchDebug Condition="'$(MtouchDebug)' == ''">False</MtouchDebug>
 		<MtouchEnableGenericValueTypeSharing Condition="'$(MtouchEnableGenericValueTypeSharing)' == ''">True</MtouchEnableGenericValueTypeSharing>
 		<MtouchFastDev Condition="'$(MtouchFastDev)' == ''">False</MtouchFastDev>
-		<MtouchHttpClientHandler Condition="'$(MtouchHttpClientHandler)' == ''">HttpClientHandler</MtouchHttpClientHandler>
+		<MtouchHttpClientHandler Condition="'$(MtouchHttpClientHandler)' == ''">NSUrlSessionHandler</MtouchHttpClientHandler>
 		<MtouchProfiling Condition="'$(MtouchProfiling)' == ''">False</MtouchProfiling>
 		<MtouchLinkerDumpDependencies Condition="'$(MtouchLinkerDumpDependencies)' == ''">False</MtouchLinkerDumpDependencies>
 		<MtouchLink Condition="'$(MtouchLink)' == ''">SdkOnly</MtouchLink>

tests/mtouch/MTouch.cs

@@ -3673,8 +3673,8 @@ public class TestApp {
 		[TestCase ("CFNetworkHandler", "CFNetworkHandler")]
 		[TestCase ("NSUrlSessionHandler", "NSUrlSessionHandler")]
 		[TestCase ("HttpClientHandler", "HttpClientHandler")]
-		[TestCase (null, "HttpClientHandler")]
-		[TestCase ("", "HttpClientHandler")]
+		[TestCase (null, "NSUrlSessionHandler")]
+		[TestCase ("", "NSUrlSessionHandler")]
 		public void HttpClientHandler (string mtouchHandler, string expectedHandler)
 		{
 			var testCode = $@"