http: return 413 instead of 400 when the request body is too large

RFC2616 says servers should return this status code when
rejecting requests that are too large.

http://www.w3.org/Protocols/rfc2616/rfc2616-sec10.html#sec10.4.14

R=bradfitz
CC=golang-dev
https://golang.org/cl/4962041
2 files changed