Return 413 in case a packet is too large

This pull-request address this issue: https://intranet.scrapinghub.com/issues/14674

This pull-request provides a solution to the problem in which mochiweb itself returns the status code 413 (please pay attention to the commit message).
I have another solution in mind, which is almost the same as this pull-request but the difference is, we should set a flag (let's say, for example, "too_large_packet" flag) or something so that we can catch it in Crawlera side, then Crawlera should decide what to do with the request.

Any ideas?