Initializes every empty number property of a payload, as defined in the schema provided, with 0.
function formatNumber (payload, schema) {
  _.forEach(schema, function(v, k) {
    if (v !== null && v._type === 'number') {
      if (typeof payload[k] === 'undefined' || payload[k] === null) {
        payload[k] = 0
      }
    }
  })
}