Take into account the request parameters when caching a LESS SSX

The LESS cache for an SSX resource currently ignores the request parameters sent to the SSX. When the SSX LESS code contains some Velocity scripting which uses such request parameters, the only way to get the expected SSX is to deactivate the cache for that resource, which results in performance issues. This issue is about computing the cache key of a LESS resource by taking into account the provided request parameters, if any.