Request body: block write events while reading body.

Request body: block write events while reading body.

If write events are not blocked, an extra write event might happen for
various reasons (e.g. as a result of a http pipelining), resulting in
incorrect body being passed to a post handler.

The problem manifested itself with the dav module only, as this is
the only module which reads the body from a content phase handler (in
contrast to exclusive content handlers like proxy).  Additionally, dav
module used to dump core in such situations due to ticket #238.

See reports here:

http://mailman.nginx.org/pipermail/nginx-devel/2012-November/002981.html
http://serverfault.com/questions/449195/nginx-webdav-server-with-auth-request