Update from 1.15.8 to 1.15.9.

<ChangeLog>

*) Feature: variables support in the "ssl_certificate" and

"ssl_certificate_key" directives.

*) Feature: the "poll" method is now available on Windows when using

Windows Vista or newer.

*) Bugfix: if the "select" method was used on Windows and an error

occurred while establishing a backend connection, nginx waited for
the connection establishment timeout to expire.

*) Bugfix: the "proxy_upload_rate" and "proxy_download_rate" directives

in the stream module worked incorrectly when proxying UDP datagrams.

</ChangeLog>