geth should accept application/json-rpc as Content-Type

[ligi]

currently when using the content-type application/json-rpc
then the geth rpc server answers with 415 (Unsupported Media Type)

But this SHOULD be the Content-Type for json rpc as in: https://www.jsonrpc.org/historical/json-rpc-over-http.html#http-header

[ligi]

assigned to @holiman as offered in the standup