Describe the bug
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When configuring the proxy ( version 0.13.1) with :

            downloadURL =  "https://proxy.golang.org"
            mode =  "redirect"

I got lots of error messages in log like:

ERROR[2:39PM]: redirect	http-method=GET http-path=/gopkg.in/gorp.v1/@v/v1.7.2.zip request-id=613a7b67-3b2f-4814-a293-73bd370c4088
INFO[2:39PM]: incoming request	http-method=GET http-path=/gopkg.in/gorp.v1/@v/v1.7.2.zip http-status=301 request-id=613a7b67-3b2f-4814-a293-73bd370c4088

while in logs of go download i see:

# get https://athens.example.com/gopkg.in/gorp.v1/@v/v1.7.2.mod: 200 OK (0.062s)

meaning it successfully downloaded the package.

So, what does the ERROR message mean in log ?

To Reproduce
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Expected behavior
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Environment (please complete the following information):

• OS: [e.g. Linux 64bit] Linux
• Go version :
• Proxy version : 0.13.1
• Storage (fs/mongodb/s3 etc.) : s3

Additional context
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