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

Negative rb->rest can't happen with current code, but it's good to have
it handled anyway.

Found by Coverity (CID 744846, 744847, 744848).

While discarding chunked request body in some cases after detecting
request body corruption no error was returned, while it was possible
to correctly return 400 Bad Request.  If error is detected too late,
make sure to properly close connection.

Additionally, in ngx_http_special_response_handler() don't return body
of 500 Internal Server Error to a client if ngx_http_discard_request_body()
fails, but disable keepalive and continue.

Even if there is no preread data, make sure to always call
ngx_http_discard_request_body_filter() in case of chunked request
body to initialize r->headers_in.content_length_n for later use.

This allows to handle requests with chunked body by scgi module, and
also simplifies handling of various request body modifications.

This allows to handle requests with chunked body by fastcgi and uwsgi
modules, and also simplifies handling of various request body modifications.

This allows to handle requests with chunked body, and also simplifies
handling of various request body modifications.