Upstream: fixed ngx_http_upstream_test_next() conditions. Previously, ngx_http_upstream_test_next() used an outdated condition on whether it will be possible to switch to a different server or not. It did not take into account restrictions on non-idempotent requests, requests with non-buffered request body, and the next upstream timeout. For such requests, switching to the next upstream server was rejected later in ngx_http_upstream_next(), resulting in nginx own error page being returned instead of the original upstream response.