Fix details summary polyfill
Fix details summary polyfill

This was causing an error in IE7 and below.

TFOH/govuk_elements,alphagov/govuk_elements,alphagov/govuk_elements,gavboulton/govuk_elements,gavboulton/govuk_elements,gavboulton/govuk_elements,TFOH/govuk_elements,gavboulton/govuk_elements,alphagov/govuk_elements,joelanman/govuk_elements,alphagov/govuk_elements,joelanman/govuk_elements