improper Content-Type setting on JsonRpc client

[dynamicgl]

Hi I noticed there is a problem in the last commit c4ae24d

•    d = fetch(
  

  •        self.__rpcUrl,
    

  •        method="POST",
    

  •        postdata=q,
    

  •        headers={
    

  •            "Content-Type": "application/json-rpc"
    

  •        }
    

  •    )
    

"Content-Type": "application/json-rpc": ought to be

"Content-Type": ["application/json-rpc"]

otherwise , I got exception.

Thanks.