use for loop in _.each even if Array.prototype.forEach is available

Fixes #1524

For the record, I'd prefer to see us achieve consistency by moving in the *other* direction (by matching the behaviour of Array.prototype.forEach in < ES5 environments). @jashkenas has expressed interest in *this* change, though, so here it is. :)

Commit message:

> Currently, the behaviour of _.each depends on the availability of Array.prototype.forEach, which can cause code to behave differently in different environments.