title: Flurl.Http PostMultipartAsync caused some errors
body: <p>I'm posting some data to a server. This piece of code works fine a few months ago. But now it threw “Flurl.Http.FlurlHttpException”.</p>
  
  <p>Flurl.Http 2.3.2</p>
  
  <pre><code>string res = "";
  try
  {
     var obj = new { api_key = "key", api_secret = "secret", start = 1 };
     res = await "https://api-cn.faceplusplus.com/facepp/v3/faceset/getfacesets"
           .PostMultipartAsync(mp =&gt; mp
           .AddStringParts(obj)
           ).ReceiveString();
  
  }
  catch (FlurlHttpException ex)
  {
      Debug.WriteLine(ex.Message);
  }
  </code></pre>
  
  <p>The only difference is Win10 version, from 1803 to 1809.
  I changed to use "PostUrlEncodedAsync", the server returns success.
  But this method can't add a file.</p>
  
  <pre><code>res = await apiUrl
      .PostUrlEncodedAsync(obj)
      .ReceiveString();
  </code></pre>
  
  <p>Here is the FlurlHttpException:</p>
  
  <p><a href="https://i.stack.imgur.com/FJUU4.jpg" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/FJUU4.jpg" alt="enter image description here"></a></p>
  
  <pre><code>{Flurl.Http.FlurlHttpException: Call failed with status code 400 (): POST https://api-cn.faceplusplus.com/facepp/v3/faceset/getfacesets
     at Flurl.Http.FlurlRequest.&lt;HandleExceptionAsync&gt;d__23.MoveNext()
  --- End of stack trace from previous location where exception was thrown ---
     at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task)
     at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
     at System.Runtime.CompilerServices.TaskAwaiter.ValidateEnd(Task task)
     at Flurl.Http.FlurlRequest.&lt;SendAsync&gt;d__19.MoveNext()
  --- End of stack trace from previous location where exception was thrown ---
     at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
     at Flurl.Http.FlurlRequest.&lt;SendAsync&gt;d__19.MoveNext()
  --- End of stack trace from previous location where exception was thrown ---
     at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task)
     at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
     at Flurl.Http.HttpResponseMessageExtensions.&lt;ReceiveString&gt;d__3.MoveNext()
  --- End of stack trace from previous location where exception was thrown ---
     at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task)
     at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
     at System.Runtime.CompilerServices.TaskAwaiter`1.GetResult()
     at FacePlusPlusUWP.FaceServiceClient.&lt;SendRequestAsync&gt;d__26.MoveNext()}
  </code></pre>