fix: allow application/json-rpc http content type

[dan-starkware]

Hi, consider allowing 'application/json-rpc' content type.

I came across this (deprecated?) JSON-RPC over HTTP specification file that indicates that content-type should be 'application/json-rpc'.
Also, several widely used libraries, such as jsonrpc4j use it.

[niklasad1]

Fair enough, I see no problem by supporting these content types.

The assumption from our side is that "application/json" is mainly used for JSON-RPC v2 but it's not really specified there.

[lexnv]

Thanks for contributing!