Merge #20055: rpc: Set HTTP Content-Type in bitcoin-cli

7eab781a14 rpc: Set HTTP Content-Type in bitcoin-cli (Wladimir J. van der Laan)

Pull request description:

  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 at all for a POST request.

  This came up in #18950.

ACKs for top commit:
  promag:
    ACK 7eab781a14.
  jonatack:
    Tested ACK 7eab781a14
  practicalswift:
    ACK 7eab781a145a35d0373c4ab4d237a82b4919e88d: patch looks correct
  fanquake:
    ACK 7eab781a14 - Looks fine to me.

Tree-SHA512: a9fa155324d0f7bff955585a336ead6bb60b721039f424521a435e4bb0fad3f4532e5cc7b7a9acc4e93585e8d3db3082c010138810f22c0e92b8f749b86ef653

src/bitcoin-cli.cpp

@@ -596,6 +596,7 @@ static UniValue CallRPC(BaseRequestHandler* rh, const std::string& strMethod, co
     assert(output_headers);
     evhttp_add_header(output_headers, "Host", host.c_str());
     evhttp_add_header(output_headers, "Connection", "close");
+    evhttp_add_header(output_headers, "Content-Type", "application/json");
     evhttp_add_header(output_headers, "Authorization", (std::string("Basic ") + EncodeBase64(strRPCUserColonPass)).c_str());
 
     // Attach request data