Read Json stream as UTF-8

Do not (implicitly) rely on Charset.defaultCharset(). RFC 8259
specifies UTF-8 unless in a closed system. Older RFC 7159 and RFC
4627 allowed UTF-8, UTF-16, UTF-32, big or little endian, with or
without BOM.

Change-Id: I33ddfacccc97f81d9eeaf444f734c469028c8214
Signed-off-by: Thomas Wolf <thomas.wolf@paranor.ch>