should not modify options

[lianzhao]

May I know why you deleted three properties from the opts at Lin52?

[vagusX]

These three opts including 'logs', 'rewrite', 'events' are not the options for node-http-proxy and only used by koa-proxies itself.
And what's more, I am considering to split options into koa-proxies own options and node-http-proxy options. Aiming at more clear options.

[lianzhao]

I think the opts object is provided by user and does not expected to be modified