Unexpected behaviour when matrix column name includes space #255

Description

• the front end sends the name a space
• the back end changes this to a_space

Reproduce

• You can see this by downloading the XLSForm from KPI
• This is the version of the form that gets deployed to KoBoCAT and used for data collection
• Verify by downloading the deployed XLSForm from KoBoCAT via the API, e.g. https://kc.kobotoolbox.org/api/v1/forms/718575/form.xls
• Somehow, when exporting, KPI/formpack are still looking for the original a space name
• Export succeeds, but excludes data under column with space in name

Related issues

kobotoolbox/kpi#3326