Add `Hash#map_values` to ActiveSupport to simplify a common pattern

Keys remain the same, but the values change.

Didn't get a chance to convert existing code, I'll skim through the code base to make use of this later this afternoon.
Updated not to use `dup`. Opted not to copy `default`, `default_proc`, etc to be consistent with Ruby. Ruby only copies them on `reject`, but not `select`, currently, and that behavior is deprecated (will be removed in 2.2).