Json : Fix String Writer

Tidy-up and correct logic around writing JSON
encoded strings.

related #13832

Change-Id: I1eca33e11dff4457f85a896c02331c1cd9ae1110
Reviewed-on: https://review.haiku-os.org/617
Reviewed-by: waddlesplash <waddlesplash@gmail.com>

Implementation of JSON Streaming Parser

Addition of missing files from previous commit