tests: Check Content-Type header returned from RPC server
tests: Check Content-Type header returned from RPC server

Check the Content-Type header that is returned from the RPC server. Only
if it is `application/json` the data is supposed to be parsed as JSON.

This gives better reporting if the HTTP server happens to return an error that is
not JSON-formatted, which is the case if it happens at a lower level
before JSON-RPC kicks in.

Before: `Unexpected exception caught during testing: No JSON object could be decoded`

After: `JSONRPC error: non-JSON HTTP response with '403 Forbidden' from server`

bitcoinxt/bitcoinxt,dagurval/bitcoinxt,dagurval/bitcoinxt,bitcoinxt/bitcoinxt,bitcoinxt/bitcoinxt,bitcoinxt/bitcoinxt,dagurval/bitcoinxt,dagurval/bitcoinxt,bitcoinxt/bitcoinxt,bitcoinxt/bitcoinxt,dagurval/bitcoinxt,dagurval/bitcoinxt