callout with duplicated headers

callout_dupe_header.json

{
	"name": "request-callout",
	"config": {
		"callouts": [
			{
				"name": "echo",
				"cache": {
					"bypass": false
				},
				"depends_on": [],
				"request": {
					"url": "http://localhost:8080/anything",
					"method": "GET",
					"headers": {
						"custom": {
							"Content-Type": "application/application/json"
						},
						"forward": false
					},
					"body": {
						"custom": {
							"something": "foobar"
						},
						"forward": true
					},
					"by_lua": "local headers = kong.ctx.shared.callouts.echo.request.params.headers; headers['content-type'] = nil; headers['Content-Type'] = 'application/x-www-form-urlencoded;charset=utf-8'; kong.log.inspect.on(); kong.log.inspect(kong.ctx.shared.callouts.echo.request.params.headers)"
				}
			}
		]
	}
}