Tweak jsonObjectDecodeClass so that it preserves classnames

Tweak jsonObjectDecodeClass so that it preserves classnames
that are already present.  The previous version would add
new classnames, if it found them encoded, but would drop
any old ones.

At present this change will have no effect.  Apart from a
couple of recursive calls, the only function that calls
jsonObjectDecodeClass() is jsonParseString().  In that case,
we pass a freshly parsed jsonObject that can't possibly
have any classnames yet.

However this change will enable us to use this function
elsewhere (specifically in osrf_message.c), resulting in
simpler and faster code.

M    src/libopensrf/osrf_json_tools.c

git-svn-id: svn://svn.open-ils.org/OpenSRF/trunk@1805 9efc2488-bf62-4759-914b-345cdb29e865