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 with copying defaults, average performance gain is about ~15% now.

Arguably using `.dup` is more friendly to custom hash subclasses that add other behavior and rely on `.dup` to clone it. 

Whether .dup is used or not, there should also be tests for the disrepancy you caught (ensuring defaults are copied over when using `transform_values`.