title: StringContent results in null value but FormUrlEncodedContent works correctly
body: <p>I have the following model in an ASP.NET Core 2.0 Web API</p>
  
  <pre><code>public class LoginModel
  {
      public string Username { get; set; }
      public string Password { get; set; }
  }
  </code></pre>
  
  <p>Which contains the following action</p>
  
  <pre><code>[HttpPost]
  public async Task&lt;IActionResult&gt; Login(LoginModel model)
  {
      if (ModelState.IsValid)
      {
          var result = await SignInManager.PasswordSignInAsync(model.Username, model.Password, false, false);
          if (result.Succeeded)
          {
              ApplicationUser user = await UserManager.FindByNameAsync(model.Username);
              return new ObjectResult(await GenerateToken(user));
          }
      }
      return BadRequest(model);
  }
  </code></pre>
  
  <p>When using StringContent with JsonConvert in a test client this results in NULL values showing up in the model when posted to the action on the controller.</p>
  
  <pre><code>var credentials = new LoginModel() { Username = "me@me.com", Password = "somePassword" };
  var content = new StringContent(JsonConvert.SerializeObject(credentials), Encoding.UTF8, "application/json");        
  var response = await client.PostAsync("/api/auth/login", content);
  </code></pre>
  
  <p>When using FormUrlEncodedContent, the model is populated correctly in the action on the controller.</p>
  
  <pre><code>var content = new FormUrlEncodedContent(new[]
  {
       new KeyValuePair&lt;string, string&gt;("username", "me@me.com"),
       new KeyValuePair&lt;string, string&gt;("password", "somePassword"),
  }
  var response = await client.PostAsync("/api/auth/login", content);
  </code></pre>
  
  <p>I've also tried using the HttpClient extensions which also results in no values showing up in the model when posted.</p>
  
  <pre><code>var credentials = new LoginModel() { Username = "me@me.com", Password = "somePassword" };
  var response = await client.PostAsJsonAsync&lt;LoginModel&gt;("/api/auth/login", credentials);
  </code></pre>
  
  <p>What am I missing?  Any help would be greatly appreciated.</p>