CDRIVER-1335 bson_as_json for "code" types

bson_append_code_with_scope now preserves the "code with scope" type
if scope is an empty, non-NULL BSON document.

bson_as_json now properly formats BSON "code" and "code with scope"
types like '{"$code": "...", "$scope": {}}'. Before, it had treated the
code as a string and ignored the scope.