Fix: split XFF header only by comma

[osavchenko]

When splitting X-Forwarded-For header by ', ' (comma and space) I got an issue when valid value 127.0.0.1,127.0.1.1 can't be parsed correctly. I'm suggesting splitting just by , (comma) and trim a result.

[codecov]

Codecov Report

Merging #1878 (2defe74) into master (2acb24a) will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #1878      +/-   ##
==========================================
+ Coverage   90.21%   90.22%   +0.01%     
==========================================
  Files          31       31              
  Lines        2770     2773       +3     
==========================================
+ Hits         2499     2502       +3     
  Misses        173      173              
  Partials       98       98              

Impacted Files	Coverage Δ
context.go	87.19% <100.00%> (ø)
bind.go	89.41% <0.00%> (+0.19%)	⬆️

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[aldas]

It seems that a lot of sites say that comma+space is used. But https://datatracker.ietf.org/doc/html/rfc7239#section-7.1 seems to say that

Note that an HTTP list allows white spaces to occur between the
identifiers, and the list may be split over multiple header fields.
As an example, the header field

so I'll merge this pr.