fix: Return string instead of JSON content

src/Docker.DotNet/JsonRequestContent.cs

@@ -23,6 +23,6 @@ public JsonRequestContent(T val, JsonSerializer serializer)
 
     public HttpContent GetContent()
     {
-        return _serializer.GetJsonContent(_value);
+        return _serializer.GetHttpContent(_value);
     }
 }

src/Docker.DotNet/JsonSerializer.cs

@@ -4,6 +4,10 @@ internal sealed class JsonSerializer
 {
     private readonly JsonSerializerOptions _options = new JsonSerializerOptions();
 
+    static JsonSerializer()
+    {
+    }
+
     private JsonSerializer()
     {
         _options.Converters.Add(new JsonEnumMemberConverter<RestartPolicyKind>());
@@ -16,9 +20,9 @@ private JsonSerializer()
     public static JsonSerializer Instance { get; }
         = new JsonSerializer();
 
-    public HttpContent GetJsonContent<T>(T value)
+    public HttpContent GetHttpContent<T>(T value)
     {
-        return JsonContent.Create(value, options: _options);
+        return new StringContent(Serialize(value), Encoding.UTF8, "application/json");
     }
 
     public string Serialize<T>(T value)

version.json

@@ -1,6 +1,6 @@
 {
   "$schema": "https://raw.githubusercontent.com/dotnet/Nerdbank.GitVersioning/master/src/NerdBank.GitVersioning/version.schema.json",
-  "version": "3.128.3",
+  "version": "3.128.4",
   "nugetPackageVersion": {
     "semVer": 2
   },