rpc: Set HTTP Content-Type in bitcoin-cli

Summary:

We don't set any Content-Type in the client. It is more
consistent with our other JSON-RPC use to set it to application/json.

Note that our server doesn't enforce content types, so it doesn't make a
difference in practice. But it is fairly strange HTTP behavior to not set it.

This came up in #18950.

This is a backport of core#20055

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, majcosta

Reviewed By: #bitcoin_abc, majcosta

Differential Revision: https://reviews.bitcoinabc.org/D10419