title: Unable to unit test simple endpoint with Carter in C Sharp
body: <p>Im trying to unit test my REST web api coded in C Sharp <strong>Carter framework</strong> using <strong>ASP.NET TestHost Version 2.1</strong>
  This is my endpoint :</p>
  
  <pre><code>namespace Api
  {
      public class HealthModule : CarterModule
      {
          public HealthModule()
          {
              Get("/health", async (req, res, routeData) =&gt; await res.WriteAsync("I'm healthy!"));
          }
      }
  }
  </code></pre>
  
  <p>And this is my test :</p>
  
  <pre><code>namespace Api.Tests
  {
      public class HealthModuleTests
      {
          private HttpResponseMessage response;
  
          public HealthModuleTests()
          {
              var client = new TestServer(new WebHostBuilder()
                      .ConfigureServices(services =&gt;
                      {
                          services.AddCarter();
                      })
                      .Configure(app =&gt; { app.UseCarter(); })
                  ).CreateClient();
  
              response = client.GetAsync("/health").GetAwaiter().GetResult();
          }
  
          [Fact]
          public void HealthRequestOK()
          {
              Assert.Equal(HttpStatusCode.OK, response.StatusCode);
              Assert.Equal("I'm healthy!", response.ReasonPhrase);
          }
      }
  }
  </code></pre>
  
  <p>Note : My app is a NetCore 2.1 app.</p>
  
  <p>Error : </p>
  
  <pre><code>Result Message: 
  System.Reflection.ReflectionTypeLoadException : Unable to load one or more of the requested types.
  Could not load file or assembly 'Microsoft.Net.Http.Headers, Version=2.1.1.0, Culture=neutral, PublicKeyToken=adb9793829ddae60'. The system cannot find the file specified.
  </code></pre>
  
  <p>What can be causing the above error?</p>