Resolve header references on read

Fixes #515

Author: brantburnett

src/Microsoft.OpenApi.Readers/Services/OpenApiReferenceResolver.cs

@@ -52,6 +52,7 @@ public override void Visit(OpenApiComponents components)
             ResolveMap(components.Examples);
             ResolveMap(components.Schemas);
             ResolveMap(components.SecuritySchemes);
+            ResolveMap(components.Headers);
         }
 
         public override void Visit(IDictionary<string, OpenApiCallback> callbacks)
@@ -99,6 +100,14 @@ public override void Visit(OpenApiResponses responses)
             ResolveMap(responses);
         }
 
+        /// <summary>
+        /// Resolve all references to headers
+        /// </summary>
+        public override void Visit(OpenApiResponse response)
+        {
+            ResolveMap(response.Headers);
+        }
+
         /// <summary>
         /// Resolve all references to SecuritySchemes
         /// </summary>

test/Microsoft.OpenApi.Readers.Tests/Microsoft.OpenApi.Readers.Tests.csproj

@@ -150,6 +150,7 @@
         <CopyToOutputDirectory>Never</CopyToOutputDirectory>
       </EmbeddedResource>
       <EmbeddedResource Include="V3Tests\Samples\OpenApiExample\explicitString.yaml" />
+      <EmbeddedResource Include="V3Tests\Samples\OpenApiResponse\responseWithHeaderReference.yaml" />
       <EmbeddedResource Include="V3Tests\Samples\OpenApiInfo\basicInfo.yaml">
         <CopyToOutputDirectory>Never</CopyToOutputDirectory>
       </EmbeddedResource>

test/Microsoft.OpenApi.Readers.Tests/V3Tests/OpenApiResponseTests.cs

@@ -0,0 +1,33 @@
+// Copyright (c) Microsoft Corporation. All rights reserved.
+// Licensed under the MIT license.
+
+using System.IO;
+using System.Linq;
+using FluentAssertions;
+using Microsoft.OpenApi.Any;
+using Microsoft.OpenApi.Models;
+using Microsoft.OpenApi.Readers.ParseNodes;
+using Microsoft.OpenApi.Readers.V3;
+using Xunit;
+
+namespace Microsoft.OpenApi.Readers.Tests.V3Tests
+{
+    [Collection("DefaultSettings")]
+    public class OpenApiResponseTests
+    {
+        private const string SampleFolderPath = "V3Tests/Samples/OpenApiResponse/";
+
+        [Fact]
+        public void ResponseWithReferencedHeaderShouldReferenceComponent()
+        {
+            using (var stream = Resources.GetStream(Path.Combine(SampleFolderPath, "responseWithHeaderReference.yaml")))
+            {
+                var openApiDoc = new OpenApiStreamReader().Read(stream, out var diagnostic);
+
+                var response = openApiDoc.Components.Responses["Test"];
+
+                Assert.Same(response.Headers.First().Value, openApiDoc.Components.Headers.First().Value);
+            }
+        }
+    }
+}

test/Microsoft.OpenApi.Readers.Tests/V3Tests/Samples/OpenApiResponse/responseWithHeaderReference.yaml

@@ -0,0 +1,16 @@
+openapi: 3.0.0
+info:
+  title: Example of response referencing a header
+  version: 1.0.0
+components:
+  headers:
+    X-Test:
+      description: Test
+      schema:
+        type: string
+  responses:
+    Test:
+      description: Test Repsonse
+      headers:
+        X-Test:
+          $ref: '#/components/headers/X-Test'