handleMixins should iterate forward when invoking mixin.mixconfig

invoking mixin.mixconfig while iterating backwards over resolvedMixins for an instance does not allow modification of a mixin's configuration from a downstream mixin. This patch fixes it.
Whenever using `_.extend` on a variable called defaults I ask why not use `_.defaults`. What if we were to do something like `_.extend instance.mixinOptions, _.defaults(mixinOptions, mixinDefaults)`. Does that produce the same effect? Then we keep modifying the options object instead of overwriting it (with a modified copy of itself) for each mixin.