Use UTF-8 instead of ASCII for setting object properties from folly::dynamic

Summary:
Based on Hermes Issue: https://github.com/facebook/hermes/issues/47
It was not actually a bug in Hermes, but a bug in JSI, assuming that all string property names from
`folly::dynamic` are ASCII.

Now we'll be more intentional and directly state `forUtf8` rather than the implicit ASCII encoding.

Reviewed By: mhorowitz

Differential Revision: D16347857

fbshipit-source-id: 6bcfbf9f918dc0a7a485b88a1b537d6c2dd322cc