Changing Host value in HTTP header doesn't work

Hello,

In my current project we have to test a REST service that require a specific value to be set in the Host value of the HTTP header.
In the definition of the resource we have added the HEADER parameter 'Host' with the value required.
It seems that the value in the header is not changed. It remains the hostname specified as endpoint for the service.

For other values of the header it works properly.

Any clue?

Thank you