Override ContentType header when sending multipart/form-data type request (#14925)

Author: ShivangiReja

sdk/core/Azure.Core/src/Shared/Multipart/MultipartContent.cs

@@ -112,7 +112,8 @@ private static string GetDefaultBoundary()
         /// <param name="request">The request.</param>
         public void ApplyToRequest(Request request)
         {
-            request.Headers.Add(HttpHeader.Names.ContentType, $"multipart/{_subtype}; boundary={_boundary}");
+            request.Headers.SetValue(HttpHeader.Names.ContentType, $"multipart/{_subtype}; boundary={_boundary}");
+            request.Content = this;
         }
 
         /// <summary>

sdk/core/Azure.Core/tests/HttpPipelineFunctionalTests.cs

@@ -404,7 +404,6 @@ public async Task SendMultipartformData()
             request.Uri.Reset(testServer.Address);
 
             var content = new MultipartFormDataContent("test_boundary");
-            content.ApplyToRequest(request);
             content.Add(RequestContent.Create(Encoding.UTF8.GetBytes("John")), "FirstName", "file_name.txt", new Dictionary<string, string>
             {
                 { "Content-Type", "text/plain; charset=utf-8" }
@@ -414,7 +413,7 @@ public async Task SendMultipartformData()
                 { "Content-Type", "text/plain; charset=utf-8" }
             });
 
-            request.Content = content;
+            content.ApplyToRequest(request);
 
             using Response response = await ExecuteRequest(request, httpPipeline);
             Assert.AreEqual(response.Status, 200);

sdk/core/Azure.Core/tests/MultipartTests.cs

@@ -184,7 +184,6 @@ public async Task SendMultipartData()
 
             Guid batchGuid = Guid.NewGuid();
             var content = new MultipartContent(Mixed, $"batch_{batchGuid}");
-            content.ApplyToRequest(request);
 
             Guid changesetGuid = Guid.NewGuid();
             var changeset = new MultipartContent(Mixed, $"changeset_{changesetGuid}");
@@ -228,7 +227,7 @@ public async Task SendMultipartData()
             patchReq.Content = RequestContent.Create(Encoding.UTF8.GetBytes(patchBody));
             changeset.Add(new RequestRequestContent(patchReq), new Dictionary<string, string> { { HttpHeader.Names.ContentType, "application/http" }, { cteHeaderName, Binary } });
 
-            request.Content = content;
+            content.ApplyToRequest(request);
             var memStream = new MemoryStream();
             await content.WriteToAsync(memStream, default);
             memStream.Position = 0;