Inputs are not un-rounded on iOS in v4

Created by: davidglenn

Setting $enable-rounded: false doesn't remove rounded borders from input fields on iOS in v4. (tested on iOS 8 & 9 on iPhone 6)

iOS rounds borders by default.

mixin only rounds when $enabled-rounded == true

@mixin border-radius($radius: $border-radius) {
  @if $enable-rounded {
    border-radius: $radius;
  }
}

So I guess this should be normalized in _reboot.scss?