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`
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