Fix the EducationNavigation header name
Fix the EducationNavigation header name

The header is called `GOVUK-ABTest-EducationNavigation`. This commit
makes sure we pick up the correct value from nginx and pass it on to
kibana.

alphagov/govuk-puppet,alphagov/govuk-puppet,alphagov/govuk-puppet,alphagov/govuk-puppet,alphagov/govuk-puppet,alphagov/govuk-puppet