AssetTag#compute_public_path caching should depend on given parameter

I have some classes extending AssetTag and friends (awesome refactoring, Josh!) to allow assets to be cached to different paths depending on an active theme.

Code worked find until recently. This broke it: http://github.com/rails/rails/co...

Fix is attached. I'm aware that I'm using #compute_public_path which is not protected, but private, so probably not recommended to be used by extending classes. Nonetheless I feel that the offending line is a bug: when compute_public_path takes a parameter source then it should return different results for different sources, or shouldn't it?

Anyway, I guess the patch doesn't hurt anybody :)

I'm adding a separate patch for a test because when looking at the tests for asset_tag_helper I found that the internal api is not tested at all, so I was unsure if there even should be a test for this particular thing. I believe there should be one, so I'll attach the patch for that as well.