Header is misaligned in Internet Explorer 8

[NickColley]

I'm not sure if this is a recent regression, but I found that the header doesnt render as I'd expect while working on the spacing (#1122)

On the GOV.UK Prototype Kit site there also seems to be misrendering of the header:

In internet explorer 9 there are no issues with either.

[NickColley]

Looks to be because the container does not have margin: auto, since those rules are in a media query (which are not supporting in IE8).

We could consider adding margin: auto to the header container, or potentially this is a wider issue for all containers: https://github.com/alphagov/govuk-frontend/blob/master/src/objects/_width-container.scss#L1