API Proxy - relative url

In API Management I have created an API proxy by importing the "URLMask" example (which is available at https://github.com/SAP/cloud-api-management-samples) The problem I have is that the target site contains several relative urls that loads CSS files and Java Script files. My issue is that the proxy base URL is dropped

Anybody have experienced the same issue ?

Regards

Rasmus
Example:

Target: https://testsite.com/

Proxy Endpoint: https://trial.apim1.hanatrial.ondemand.com/ <account name>/v1/test

The response contains a relative url like /javascripts/script.js which result in a new request :

https://trial.apim1.hanatrial.ondemand.com/javascripts/script.js