Golang functions: Multi value headers not working in local development

[kasperstorgaard]

Describe the bug

When running netlify dev with golang functions, multi value headers are not working, but works in deploy preview, prod etc.

(This is specifically hindering me from testing a gin-gonic adapter setup locally, as the adapter uses multi value headers extensively)

Steps to reproduce

1. Run git clone git@github.com:kasperstorgaard/netlify-golang-fn-tester.git
2. Navigate to cloned repository
3. Run npx netlify dev OR npm i && node_modules/.bin/netlify dev
4. Open localhost:8888/.netlify/functions/tester
5. Inspect request response in tool of choice

Expected headers:

"x-single": "A"
"x-multi":  "B, C"

Actual headers:

"x-single": "A"

Configuration

[functions]
directory = "src"

Environment

System:
OS: macOS 11.6
CPU: (8) x64 Intel(R) Core(TM) i5-1038NG7 CPU @ 2.00GHz
Memory: 74.09 MB / 16.00 GB
Shell: 5.8 - /bin/zsh
Binaries:
Node: 16.14.2 - ~/.nvm/versions/node/v16.14.2/bin/node
npm: 8.5.0 - ~/.nvm/versions/node/v16.14.2/bin/npm
npmPackages:
netlify-cli: ^10.0.0 => 10.1.0
npmGlobalPackages:
netlify-cli: 10.0.0