Fixed #16850 -- Made sure we always represent Decimal instances with JS strings when serializing.

We force this when calling simplejson dump() to isolate us from changes of
default behavior to encode them with JavaScript numbers instead that could
introduce loss of precision when decoding back from JSON to Python.

Thanks Raphael Hertzog for the report and Bob Ippolito for his help.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@17228 bcc190cf-cafb-0310-a4f2-bffc1f526a37