New records clash with fixtures (if ids are integers)

When new records are added with the fixture adapter, their ids are auto-increment always starting from 0. In the case where fixtures already exist and also have integers as their ids (eg 1,2,3) a clash is inevitable. Mostly this happens when inserting the 2nd record (since the first works with id=0, the second clashes with id=1).

This pull request fixes it by changing the counter value past the highest fixture id, so that clashes won't occur. For non-number fixture ids, this change will have no effect.