setPolicy
Sets @policy to @hsts_enforcer.
If @policy is expired, any existing HSTS policy for its host will be removed instead. If a policy existed for this host, it will be replaced. Otherwise, the new policy will be inserted. If the policy is a session policy, that is, one created with ctor@HSTSPolicy.new_session_policy, the policy will not expire and will be enforced during the lifetime of @hsts_enforcer's class@Session.
Parameters
policy
the policy of the HSTS host