I just ran through the steps you've described on Kibana 5.4.3 and I'm unable to reproduce that error. Are you using X-Pack by any chance? Can you double-check and make sure you're using ES 5.4.3, too? And just to confirm, you're by "content-header" I'm assuming you're referring to the content-type header.
To give you some background, the user shouldn't be required to set the content-type header when using Console, which is why this is a bit unusual to me. I wonder if this is related to https://github.com/elastic/kibana/issues/10504. Could you make your request using CURL and let me know if it still returns the same error?
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