Feature/inject error mapper (#562)

* added delegate to select last handler

* #529 implemented a way we can inject the last delegating handler

* wip - moving code

* #529 removed loads of qos code and moved it into Ocelot.Provider.Polly

* #529 can now inject http client expcetions to ocelot errors mappers and updated docs

test/Ocelot.UnitTests/Requester/QoSFactoryTests.cs

@@ -0,0 +1,55 @@
+namespace Ocelot.UnitTests.Requester
+{
+    using System.Net.Http;
+    using Microsoft.Extensions.DependencyInjection;
+    using Moq;
+    using Ocelot.Configuration;
+    using Ocelot.Configuration.Builder;
+    using Ocelot.Logging;
+    using Ocelot.Requester;
+    using Ocelot.Requester.QoS;
+    using Shouldly;
+    using Xunit;
+
+    public class QoSFactoryTests
+    {
+        private QoSFactory _factory;
+        private ServiceCollection _services;
+        private readonly Mock<IOcelotLoggerFactory> _loggerFactory;
+
+        public QoSFactoryTests()
+        {
+            _services = new ServiceCollection();
+            _loggerFactory = new Mock<IOcelotLoggerFactory>();
+            var provider = _services.BuildServiceProvider();
+            _factory = new QoSFactory(provider, _loggerFactory.Object);
+        }
+
+        [Fact]
+        public void should_return_error()
+        {
+            var downstreamReRoute = new DownstreamReRouteBuilder().Build();
+            var handler = _factory.Get(downstreamReRoute);
+            handler.IsError.ShouldBeTrue();
+            handler.Errors[0].ShouldBeOfType<UnableToFindQoSProviderError>();
+        }
+
+        [Fact]
+        public void should_return_handler()
+        {
+            _services = new ServiceCollection();
+            DelegatingHandler QosDelegatingHandlerDelegate(DownstreamReRoute a, IOcelotLoggerFactory b) => new FakeDelegatingHandler();
+            _services.AddSingleton<QosDelegatingHandlerDelegate>(QosDelegatingHandlerDelegate);
+            var provider = _services.BuildServiceProvider();
+            _factory = new QoSFactory(provider, _loggerFactory.Object);
+            var downstreamReRoute = new DownstreamReRouteBuilder().Build();
+            var handler = _factory.Get(downstreamReRoute);
+            handler.IsError.ShouldBeFalse();
+            handler.Data.ShouldBeOfType<FakeDelegatingHandler>();
+        }
+
+        class FakeDelegatingHandler : DelegatingHandler
+        {
+        }
+    }
+}