Use a CopyOnWriteDictionary in ApplicationBuilder so branches can set their own properties

Fix #783

src/Microsoft.AspNetCore.Http/Internal/ApplicationBuilder.cs

@@ -29,7 +29,7 @@ namespace Microsoft.AspNetCore.Builder.Internal
 
         private ApplicationBuilder(ApplicationBuilder builder)
         {
-            Properties = builder.Properties;
+            Properties = new CopyOnWriteDictionary<string, object>(builder.Properties, EqualityComparer<string>.Default);
         }
 
         public IServiceProvider ApplicationServices

src/Microsoft.AspNetCore.Http/Microsoft.AspNetCore.Http.csproj

@@ -18,6 +18,7 @@
   </ItemGroup>
 
   <ItemGroup>
+    <PackageReference Include="Microsoft.Extensions.CopyOnWriteDictionary.Sources" Version="$(AspNetCoreVersion)" PrivateAssets="All" />
     <PackageReference Include="Microsoft.Extensions.ObjectPool" Version="$(AspNetCoreVersion)" />
     <PackageReference Include="Microsoft.Extensions.Options" Version="$(AspNetCoreVersion)" />
     <PackageReference Include="Microsoft.Extensions.TaskCache.Sources" Version="$(AspNetCoreVersion)" PrivateAssets="All" />

test/Microsoft.AspNetCore.Http.Tests/Internal/ApplicationBuilderTests.cs

@@ -19,5 +19,17 @@ namespace Microsoft.AspNetCore.Builder.Internal
             app.Invoke(httpContext);
             Assert.Equal(httpContext.Response.StatusCode, 404);
         }
+
+        [Fact]
+        public void PropertiesDictionaryIsDistinctAfterNew()
+        {
+            var builder1 = new ApplicationBuilder(null);
+            builder1.Properties["test"] = "value1";
+
+            var builder2 = builder1.New();
+            builder2.Properties["test"] = "value2";
+
+            Assert.Equal(builder1.Properties["test"], "value1");
+        }
     }
 }