Artifactory is causing checksum mismatch when proxying GitHub

Issue description

Artifactory is providing different checksum for module github.com/Shopify/sarama@v1.23.1 zip package compared with the one obtained when resolving module from the source directly.

Expected result

The checksums should be the same.

Steps to reproduce

• Using Go 1.13
• Set GOPROXY to direct:

  export GOPROXY=direct
  

• Clean up Go local cache:

  sudo rm -rf $GOPATH
  

• Fetch Go Module content

  go mod download -json github.com/Shopify/sarama@v1.23.1
  

• The json output contains the following checksums:

  	"Sum": "h1:XxJBCZEoWJtoWjf/xRbmGUpAmTZGnuuF0ON0EvxxBrs=",
  	"GoModSum": "h1:XLH1GYJnLVE0XCr6KdJGVJRTwY30moWNJ4sERjXX6fs="
  

• In Artifactory create a remote Go repo pointing to GitHub
• In Artifactory create a virtual repo containing the remote one created on the previous step
• Set GOPROXY to the virtual repo

  export GOPROXY="http://localhost:8082/artifactory/api/go/go"
  

• Clean up Go local cache:

  sudo rm -rf $GOPATH
  

• Fetch Go Module content

  go mod download -json github.com/Shopify/sarama@v1.23.1
  

• Confirm that the checksum for the zip file returned by Artifactory do not match the previous one

  	"Sum": "h1:blmR9XJ4lXLR63pxOfa2otGuVYnQ+qFZ7JpnpkIA3nk=",
  	"GoModSum": "h1:XLH1GYJnLVE0XCr6KdJGVJRTwY30moWNJ4sERjXX6fs="