Provide dispatcher option to ignore header changes on include

The servlet specification states in section 9.3 that on dispatcher include (not forward), all changes to headers from the include must be ignored.
As Sling is following the servlet spec for includes and forwards, Sling should do the same. However, after 15 years of allowing an include to change headers, this has a large potential to break existing applications.
Therefore we should rather provide a dispatcher option, that can be set to force this new behaviour.