fix: Stop Duplicate Response Handlers on Retries

[danielbankhead]

Previously, startUploading would setup a 'response' event listener for passing responses to responseHandler - this is problematic in the event of a retry as the retry would create its own request (thus previous requests would trigger responseHandler as well).

Instead, we can accomplish the desired functionality by checking makeRequestStream's return value.

Fixes #493 🦕

[shaffeeullah]

woo! thanks, dan!!