Ruby configuration - content-type

Hello, using Ruby (Rails) sdk (gem ‘sentry-raven’), against an onpremise docker instance.
I have connectivity working.
I am receiving 400 on all requests. Using Rails Console to set DSN and test capture :

Raven.configure {|c| c.dsn= "http://dsn_key:dsn_secret@sentry_host:9000/2" }
Raven.capture { 1/0 }

Rails log reports :

Sending event ec492... to Sentry
Unable to record event with remote Sentry server (Raven::Error - the server responded with status 400):

Wireshark on the request shows :

I believe (from other posts) that the required content type is application/json.

Could you please tell me how can I set the Ruby (Raven) configuration to force content-type to be application/json?

Thanks and Kindest Regards,
Peter Fitzgibbons