Allow a null content type for resumable uploads.

Src/Support/GoogleApis.Tests/Apis/Upload/ResumableUploadTest.cs

@@ -173,6 +173,8 @@ private class SingleChunkMessageHandler : BaseMockMessageHandler
             /// <summary>Gets or sets the path parameters which should be part of the initialize request.</summary>
             public string PathParameters { get; set; }
 
+            public string ExpectedContentType { get; set; } = "text/plain";
+
             protected override Task<HttpResponseMessage> SendAsyncCore(HttpRequestMessage request,
                 CancellationToken cancellationToken)
             {
@@ -191,8 +193,12 @@ private class SingleChunkMessageHandler : BaseMockMessageHandler
                         }
                         Assert.That(request.RequestUri.Query, Is.EqualTo("?uploadType=resumable" + QueryParameters));
 
-                        Assert.That(request.Headers.GetValues("X-Upload-Content-Type").First(),
-                            Is.EqualTo("text/plain"));
+                        // HttpRequestMessage doesn't make it terrible easy to get a header value speculatively...
+                        string actualContentType = request.Headers
+                            .Where(h => h.Key == "X-Upload-Content-Type")
+                            .Select(h => h.Value.FirstOrDefault())
+                            .FirstOrDefault();
+                        Assert.That(actualContentType, Is.EqualTo(ExpectedContentType));
                         Assert.That(request.Headers.GetValues("X-Upload-Content-Length").First(),
                             Is.EqualTo(StreamLength.ToString()));
 
@@ -609,6 +615,30 @@ public void TestUploadSingleChunk()
             Assert.That(handler.Calls, Is.EqualTo(2));
         }
 
+        [Test]
+        public void TestUploadNullContentType()
+        {
+            var stream = new MemoryStream(Encoding.UTF8.GetBytes(UploadTestData));
+            var handler = new SingleChunkMessageHandler()
+            {
+                StreamLength = stream.Length,
+                ExpectedContentType = null
+            };
+            using (var service = new MockClientService(new BaseClientService.Initializer()
+            {
+                HttpClientFactory = new MockHttpClientFactory(handler)
+            }))
+            {
+
+                int chunkSize = UploadTestData.Length + 10;
+                var upload = new MockResumableUpload(service, "", "POST", stream, null, chunkSize);
+                // Chunk size is bigger than the data we are sending.
+                upload.Upload();
+            }
+
+            Assert.That(handler.Calls, Is.EqualTo(2));
+        }
+
         /// <summary>Tests uploading a single chunk.</summary>
         [Test]
         public void TestUploadSingleChunk_ExactChunkSize()

Src/Support/GoogleApis/Apis/[Media]/Upload/ResumableUpload.cs

@@ -97,7 +97,9 @@ public class ResumableUpload<TRequest>
         /// <param name="path">The path for this media upload method.</param>
         /// <param name="httpMethod">The HTTP method to start this upload.</param>
         /// <param name="contentStream">The stream containing the content to upload.</param>
-        /// <param name="contentType">Content type of the content to be uploaded.</param>
+        /// <param name="contentType">Content type of the content to be uploaded. Some services
+        /// may allow this to be null; others require a content type to be specified and will
+        /// fail when the upload is started if the value is null.</param>
         /// <remarks>
         /// Caller is responsible for maintaining the <paramref name="contentStream"/> open until the upload is 
         /// completed.
@@ -106,11 +108,10 @@ public class ResumableUpload<TRequest>
         protected ResumableUpload(IClientService service, string path, string httpMethod, Stream contentStream,
             string contentType)
         {
-            service.ThrowIfNull("service");
-            path.ThrowIfNull("path");
-            httpMethod.ThrowIfNullOrEmpty("httpMethod");
-            contentStream.ThrowIfNull("stream");
-            contentType.ThrowIfNull("contentType");
+            service.ThrowIfNull(nameof(service));
+            path.ThrowIfNull(nameof(path));
+            httpMethod.ThrowIfNullOrEmpty(nameof(httpMethod));
+            contentStream.ThrowIfNull(nameof(contentStream));
 
             this.Service = service;
             this.Path = path;
@@ -731,7 +732,10 @@ private HttpRequestMessage CreateInitializeRequest()
             SetAllPropertyValues(builder);
 
             HttpRequestMessage request = builder.CreateRequest();
-            request.Headers.Add(PayloadContentTypeHeader, ContentType);
+            if (ContentType != null)
+            {
+                request.Headers.Add(PayloadContentTypeHeader, ContentType);
+            }
 
             // if the length is unknown at the time of this request, omit "X-Upload-Content-Length" header
             if (StreamLength != UnknownSize)