Fixed StreamableHttpClientSessionTransport.cs for netstandard

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

"application/json; charset=utf-8" is an invalid header.  the correct one is "application/json".

Author: lucapivato

src/ModelContextProtocol.Core/Client/StreamableHttpClientSessionTransport.cs

@@ -75,7 +75,7 @@ internal async Task<HttpResponseMessage> SendHttpRequestAsync(JsonRpcMessage mes
         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 static void CopyAdditionalHeaders(
             }
         }
     }
-}
+}