Added a differenciation between AssociationCollection#size and -length. Now AssociationCollection#size returns the size of the collection by executing a SELECT COUNT(*) query if the collection hasn't been loaded and calling collection.size if it has. If it's more likely than not that the collection does have a size larger than zero and you need to fetch that collection afterwards, it'll take one less SELECT query if you use length.

git-svn-id: http://svn-commit.rubyonrails.org/rails/trunk@392 5ecf4fe2-1ee6-0310-87b1-e25e094e27de