BUG - Sticky session keep-alive does not work

[drktfl]

Description

This is a follow-up issue to #725: The mockserver's router does not process HEAD requests to the service root. But these requests are used by the client to keep a session alive. The consequence is that sessions always terminate after 120s.

Steps to Reproduce

Use a Fiori elements app with a "sticky" service, leave it in edit mode for some minutes and check what happens

Expected results

HEAD <serviceroot> inside a session should reset the timeout.

Actual results

HEAD <serviceroot> is not processed at all beyond the CSRF token refresh logic