Unable to log certain environment variables in access log

[VladV]

%{FOO}e format doesn't accept underscores, so variables like SERVER_SOFTWARE cannot be included in the log.
Tested on 0.21.6, probably applies to the latest version as well.

[asvetlov]

Regex from https://github.com/KeepSafe/aiohttp/blob/master/aiohttp/helpers.py#L272 should accept underscore.
The patch is easy.
Any volunteers?

[esaezgil]

I will address this :)

[asvetlov]

Fixed by @1226

[lock]

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