Eliminate log messages resulting from requests being sent over a Unix Domain Socket

[janzantinge]

Fixes #1538

Removed the "invalid remote addr" log that's printed when a http request's remote address is not an IP:port while setting the X-Forwarded-For header. This issue arose specifically when requests arrived via an abstract Unix Domain Socket.

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[johanbrandhorst]

LGTM

[janzantinge]

@googlebot I signed it!

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[johanbrandhorst]

Thanks for your contribution! Could you please cherry-pick this fix against v2?