Mimitabby had some GREAT suggestions.
I also ride in the country where dogs are usually not penned up in yards very well. Have had numerous dogs hit the street after me running like the wind. I think they only want to chase you because you're moving fast, but it always scares the beejeebers out of me. I usually yell very loudly "Back" or "Get" and unclip one foot- because as much as I love animals, I'm not above kicking one to keep him away from my wheels. Luckily I've never had an accident b/c of an animal, but I've been pretty close. I have never squirted them with water, as I'm not that quick to react, reach down, open the spout and squirt (slow reflexes under panic I guess).
My DH is a mailman and has been attacked by dogs, birds, and even a goat- and the thing that works the most with dogs is an angry tone and a yell to let them know who's boss. I'd carry pepper spray, but with the omnipresent OK wind- it would surely backfire and squirt me in the face.
Stay safe out there!