My brother-in-law and I were riding on a "rail to trail" bike path some years back, when we approached two pedestrians with a large dog.  I've no idea what type of dog it was, but it was bigger than your average dog, with short hair, and golden color.  Reminded me of a guard dog, like a Leonberger, calm and not as tense as a German Sheperd, but very ready to protect it's owner.

Unfortunately for us, the dog backed out of it's collar (smart too), and we all faced each other in a stand off, as the owner tried to control her dog.  I was on my bike, one foot on the ground, and he charged at me, with mouth open, but not biting, although he did nick my knee with his tooth.  Then he turned his attention to my brother-in-law.  He's from a town full of stray dogs, some dangerous, so he deftly kept the bike between himself and the dog, like a lion tamer with a chair.  Each time the dog charged him, he'd move the bike to block the dog.  Very clever.

Finally the owner got the dog back under control and we went on our way.  They seemed very apologetic and embarrassed. 

After reading your post and all the useful links, next time I will be sure and get the owner's name and the dog's papers.  On the other hand, someone on the BT thread said he would kick the dog, I would recommend not taking that advice.  According to my brother-in-law, things like that (hitting the dog, etc) would only get the dog madder.  As someone pointed out, German Shepherds are very quick, for example, and obviously certain dogs are trained attack dogs and you would not stand a chance trying to kick them.

Pepper spray seems like a good idea though.