[nginx] Upstream: enable hash balancing method support parameter "backup".

[Elza Scarlet]

Dear Development Team,

Thanks all of you make nginx so nice to use. We have a use case, we use hash balancing method to increasing service's cache hit rate, but it doesn't support "backup" parameter,  which can be use to make doing online backup server easier. After reading the source code, we found it's very easy to support "backup" parameter when using hash balancing method, so we submit this changes. Hope you can accept this changes.

Kind regards, Scarlet
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