Ensure that default media type is used when 'null' is passed-in to StringContent #81506 (#83425)

Co-authored-by: Simon Lovely <[email protected]>

Author: github-actions[bot]

src/libraries/System.Net.Http/src/System/Net/Http/StringContent.cs

@@ -45,7 +45,7 @@ public StringContent(string content, Encoding? encoding)
         /// <param name="encoding">The encoding to use for the content.</param>
         /// <param name="mediaType">The media type to use for the content.</param>
         public StringContent(string content, Encoding? encoding, string mediaType)
-            : this(content, encoding, new MediaTypeHeaderValue(mediaType, (encoding ?? DefaultStringEncoding).WebName))
+            : this(content, encoding, new MediaTypeHeaderValue(mediaType ?? DefaultMediaType, (encoding ?? DefaultStringEncoding).WebName))
         {
         }
 

src/libraries/System.Net.Http/tests/FunctionalTests/StringContentTest.cs

@@ -71,6 +71,17 @@ public async Task Ctor_DefineNoEncoding_DefaultEncodingUsed()
             Assert.Equal(sourceString, roundTrip);
         }
 
+        [Fact]
+        public void Ctor_PassNullForMediaType_DefaultMediaTypeUsed()
+        {
+            string sourceString = "\u00C4\u00E4\u00FC\u00DC";
+            Encoding defaultStringEncoding = Encoding.GetEncoding("utf-8");
+            var content = new StringContent(sourceString, defaultStringEncoding, ((string)null)!);
+
+            // If no media is passed-in, the default is used
+            Assert.Equal("text/plain", content.Headers.ContentType.MediaType);
+        }
+        
         [Fact]
         public async Task Ctor_UseCustomMediaTypeHeaderValue_SpecificEncoding()
         {