BUG/MINOR: sample: fix the json converter's endian-sensitivity

BUG/MINOR: sample: fix the json converter's endian-sensitivity

About every time there's a pointer cast in the code, there's a hidden
bug, and this one was no exception, as it passes the first octet of the
native representation of an integer as a single-character string, which
obviously only works on little endian machines. On big-endian machines,
something as simple as "str(foo),json" only returns zeroes.

This bug was introduced with the JSON converter in 1.6-dev1 by commit
317e1c4f1e ("MINOR: sample: add "json" converter"), the fix may be
backported to all stable branches.

(cherry picked from commit 5715da269d6ec1e178b04d1c7aa25982e10873d7)
Signed-off-by: Christopher Faulet <cfaulet@haproxy.com>