If a client sent a POST with a huge request body, calling
req.Body.Close in the handler (which is implicit at the end of a
request) would end up consuming it all.

Put a cap on that, using the same threshold used elsewhere for similar
cases.

Fixes #9662

Change-Id: I26628413aa5f623a96ef7c2609a8d03c746669e5
Reviewed-on: https://go-review.googlesource.com/11412Reviewed-by: Andrew Gerrand <adg@golang.org>