fix for issue #26

Fix for issue #26. Finally it was easy to derive looking at the difference between src/ngx_postgres_keepalive.h and nginx src/http/modules/ngx_http_upstream_keepalive_module.c (ngx_http_upstream_keepalive_cache_t vs. ngx_postgres_keepalive_cache_t).
The key is to store the socket address in u_char sockaddr[NGX_SOCKADDRLEN] instead of struct sockaddr and memcpy where appropriate.
Please review and pull.