Fix default class on date_widget to be input-small

There was a previous commit (dd4e24324756fbb66ec2ae295cc6e49234ccce7b) which changed `input-mini` on the year to `input-small`, but I'm not sure why it wasn't applied to each of the elements.

It looks like this without:

![Screen Shot 2013-01-15 at 14 37 45](https://f.cloud.github.com/assets/101828/67928/9ffef566-5f21-11e2-91cc-a7c7085db9be.png)

And this with this patch:

![Screen Shot 2013-01-15 at 14 38 09](https://f.cloud.github.com/assets/101828/67930/a8394f42-5f21-11e2-8b82-6982ad82bf37.png)