message: fix(auth): stringify custom claims regardless of their type
  the ?? operand is used to transform any `undefined` value into `"null"`
diff: @@ -156,7 +156,7 @@ export function createHasuraJwt({
  ...Object.entries(customFields).reduce<{ [k: string]: ClaimValueType }>(
  (aggr, [key, value]) => ({
  ...aggr,
  - [`x-${kebabCase(key)}`]: value
  + [`x-${kebabCase(key)}`]: JSON.stringify(value ?? null)
  }),
  {}
  )
  