CDRIVER-1044: passing an object that undergoes default argument promotion to 'va_start' has undefined behavior

Signed-off-by: A. Jesse Jiryu Davis <jesse@mongodb.com>

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.

Signed-off-by: A. Jesse Jiryu Davis <jesse@mongodb.com>

Signed-off-by: A. Jesse Jiryu Davis <jesse@mongodb.com>