Another tweak to parse to account for another possible case.

Discovered an edge case where a problem was arising for the parse command. If you tried to trigger testmaster parse with depth > 1 and specifying a callback which outputs one or more requests which directly inherit the callback value from the initial request (i.e. request.callback = response.request.callback), then the callback value was being set to a function within parse.py (prepare_request.callback), rather than the appropriate function of the spider.