Fixed StreamableHttpClientSessionTransport.cs for netstandard (#582)

Fixed exception 'The format of value 'application/json; charset=utf-8' is invalid

src/ModelContextProtocol.Core/Client/StreamableHttpClientSessionTransport.cs

@@ -75,7 +75,7 @@ internal sealed partial class StreamableHttpClientSessionTransport : TransportBa
         using var content = new StringContent(
             JsonSerializer.Serialize(message, McpJsonUtilities.JsonContext.Default.JsonRpcMessage),
             Encoding.UTF8,
-            "application/json; charset=utf-8"
+            "application/json"
         );
 #endif
 
@@ -310,4 +310,4 @@ internal sealed partial class StreamableHttpClientSessionTransport : TransportBa
             }
         }
     }
-}
+}