Wrong content-type header in RPC responses

[sisou]

The rpcserver crate is returning application/json-rpc instead of just application/json. This is there since that crate was created.

https://github.com/nimiq/jsonrpc/blob/a089737987cdff99f1271f2f8a7fd547fdea3621/server/src/lib.rs#L220

This is messing with some service providers/exchanges automated setup.

[hrxi]

I couldn't find an official mime type for JSONRPC in the spec, so I guess we can just change to application/json if that's useful to some people.

[sisou]

@hrxi I made the commit in https://github.com/nimiq/jsonrpc, can you please make a release and use it in core-rs-albatross?