(plaintiff injured during horseback ride when thrown from horse and sued defendant for negligence; genuine issue of material fact existed concerning whether defendant had posted signage required by state Equine Activity law to preserve assumption of risk defense; assumption of risk defense not available under common law because specific law covering liability arising from equine activities applied; res ipsa loquitur inapplicable; however, summary judgment for defendant appropriate because plaintiff could not establish any evidence of negligence on defendant’s part – no evidence that defendant’s caused horse to gallop or that horse was unsafe for a novice rider or that trail negligently maintained).