(residential tenant's dog mauled and severely injured plaintiff; landlord moved for summary judgment on basis that plaintiff failed to establish that landlord had actual knowledge of dog's dangerousness; PA law requires that for landlord to be liable, landlord must have actual knowledge of dog's dangerousness; facts failed to establish that landlord never had notice of dog's dangerous propensities; immaterial under PA law that landlord should have known of dog's dangerous propensities).