Allow request IP candidates to contain ports

[Nextra]

This changes the flow of ipFromRequest to run all remoteIP candidates through net.SplitPortHost (if applicable) regardless of source, instead of treating http.Request.RemoteAddr as the lone special case.

The motivation behind this is that Azure Application Gateways add the X-Forwarded-For header as a comma separated list of IP:port instead of each entry just containing the IP address (ref), making it unnecessarily difficult to deploy echoip as-is to Azure Web App.

[mpolden]

Format with make fmt, then this should be good to go.

[Nextra]

I believe that is what I did initially, which caused all the comments on the old tests to get lined up with the new, longer lines.

[Nextra]

Separating the tests with an empty line seems to make go fmt happy with mixed comment alignments. Hope this is okay.

[deovero-jeroen]

I think there is a bug in this change.
I get this error now:

address 2a02:a474:3766:1:786c:befc:2130:ecf4: too many colons in address

while my IP 2a02:a474:3766:1:786c:befc:2130:ecf4 is valid.

[deovero-jeroen]

Reverted to 861f17b which fixed the problem.