Updates to OSAPI sizelimit middleware.

Updates the OSAPI sizelimit middleware so that we use avoid calling
len on a request body which could cause a really large request
to get buffered into memory.

Also updates the middleware to return HTTP 413 which is a more
correct error code in this case (previously it returned just 400).

Fixes LP Bug #1131857.

Change-Id: Iff6cb0c24bc21e5a1d5dd4cf29acb0b4ee221708
(cherry picked from commit 3fe81851bab39a1466d8744b372b5a287b4db93d)
Merge "Updates to OSAPI sizelimit middleware." into milestone-proposed