hubspot: fixing existing contacts race condition

Periodically if a developer called identify twice in quick succession,
we'd make two 'create contact' calls to hubspot. This fixes that race
condition by retrying a failed call.

If hubspot returns a `409` error code (`CONTACT_EXISTS`), the second
identify call will then attempt to update the existing contact instead
of creating a new one.
hubspot: adding json-stringifying to object values
0.1.19

  43fa23e  hubspot: adding json-stringifying to object values
  7773aa6  hubspot: fixing existing contacts race condition