set_policy
Description:
Sets policy to this.
 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 
      HSTSPolicy.session_policy, the policy will not
      expire and will be enforced during the lifetime of this's 
      Session.
Parameters:
| this | |
| policy | the policy of the HSTS host |