Fix HttpClient on Unix handling of 0-len content with chunked upload

Author: stephentoub

src/System.Net.Http/src/System/Net/Http/Unix/CurlHandler.EasyRequest.cs

@@ -97,6 +97,10 @@ internal void InitializeCurl()
                     SetCurlOption(CURLoption.CURLOPT_VERBOSE, 1L);
                 }
 
+                // Before actually configuring the handle based on the state of the request,
+                // do any necessary cleanup of the request object.
+                SanitizeRequestMessage();
+                
                 // Configure the handle
                 SetUrl();
                 SetNetworkingOptions();
@@ -245,6 +249,15 @@ public void Cleanup() // not called Dispose because the request may still be in
                 _sendTransferState?.Dispose();
             }
 
+            private void SanitizeRequestMessage()
+            {
+                // Make sure Transfer-Encoding and Content-Length make sense together.
+                if (_requestMessage.Content != null)
+                {
+                    SetChunkedModeForSend(_requestMessage);
+                }
+            }
+
             private void SetUrl()
             {
                 Uri requestUri = _requestMessage.RequestUri;
@@ -692,8 +705,6 @@ internal void SetRequestHeaders()
                 // Add content request headers
                 if (_requestMessage.Content != null)
                 {
-                    SetChunkedModeForSend(_requestMessage);
-
                     AddRequestHeaders(_requestMessage.Content.Headers, slist);
 
                     if (_requestMessage.Content.Headers.ContentType == null)

src/System.Net.Http/tests/FunctionalTests/PostScenarioTest.cs

@@ -69,6 +69,14 @@ public async Task PostEmptyContentUsingChunkedEncoding_Success(Uri serverUri)
                 useContentLengthUpload: false, useChunkedEncodingUpload: true);
         }
 
+        [OuterLoop] // TODO: Issue #11345
+        [Theory, MemberData(nameof(EchoServers))]
+        public async Task PostEmptyContentUsingConflictingSemantics_Success(Uri serverUri)
+        {
+            await PostHelper(serverUri, string.Empty, new StringContent(string.Empty),
+                useContentLengthUpload: true, useChunkedEncodingUpload: true);
+        }
+
         [OuterLoop] // TODO: Issue #11345
         [Theory, MemberData(nameof(EchoServers))]
         public async Task PostUsingContentLengthSemantics_Success(Uri serverUri)