LP#1285915: document that perl2JSON doesn't order hash keys

LP#1285915: document that perl2JSON doesn't order hash keys

This patch documents that JSON strings returned by
OpenSRF::Utils::JSON->perl2JSON() should not be expected to have
hash keys sorted in any particular order.  It also adjusts a
corresponding test case to consistently pass under Perl 5.18, which
introduces hash order randomization.

Forcing JSON output to be in canonical form is another option, and
easily done by JSON::XS, but would add overhead.

Signed-off-by: Galen Charlton <gmc@esilibrary.com>