guard cross-fetch by platform

[joshmsamuels]

This SDK uses browser-or-node to set the user-agent based on the javascript environment being run, however the same logic is not applied to cross-fetch. From my understanding that is because cross-fetch should be platform agnostic.

It looks like cross-fetch tries using XmlHttpRequest to make API calls when the environment is set to web worker

[bzmw]

Heyo,
Thanks for raising this. We are waiting on lquixada/cross-fetch#78 to be completed before we can close this out. Looks like the open source cogs are turning to get us there.