bug fix for input group sizes and not being the first or last child

Found this awesome less file and went to use it only to find a small bug. When using different sized input groups where the typeahead element is not the first, last, or only child of the group, the typeahead input will have a border-radius on every corner.

Eg, the left and right are `.input-group-sm` and `.input-group-lg` respectfully and if you look closely, you can see the text input has all rounded corners:
![2014-01-26--1390745995_1126x72_scrot](https://f.cloud.github.com/assets/94542/2004938/a1117f5c-86c9-11e3-9d41-f8345c03c18c.png)

This should hopefully fix that.