Echo service response does not contain contant-type header after the U2 update

[GDLMadushanka]

Description: Content-type header is not sent on the response of the echo service of Enterprise Integrator, after version 6.6.0.90

Suggested Labels: Regression

Affected Product Version: EI 6.0.0.90

OS, DB, other environment details and versions:

Steps to reproduce: 1 - on passthrough confiiguration, add the property: http.response.headers.preserve=content-type 2 - Restart the Enterprise Integrator and execute the echo sample service. 3 - Note that on the wire logs, the header isn't shown. On version 6.6.0.89, the header was there: [2022-03-25 17:46:04,144] DEBUG {org.apache.synapse.transport.http.wire} - HTTP-Listener I/O dispatcher-1 << "Content-Type: application/soap+xml; charset=UTF-8; action="urn:echoIntResponse"[\r][\n]"

Related Issues: https://github.com/wso2/product-apim/issues/7575