Response headers sharing the same names get converted into a comma-separated list

[jkbrzt]

Checklist

• I've searched for similar issues.
• I'm using the latest version of HTTPie.

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Minimal reproduction code and steps

Make a request to URL that returns repeated headers lines. Here’s an example express server that does that:

// server.js
const express = require('express')
const fs = require('fs')

const app = express()
const port = 4000

app.get('/', (req, res) => {
  res.set('x-foo', ['bar', 'baz'])
  res.send('Hello World!')
})

app.listen(port, () => {
  console.log(`Example app listening at http://localhost:${port}`)
})

The run node server.js and get the response:

http :4000

Current result

x-foo: bar, baz

Expected result

x-foo: bar
x-foo: baz

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@isidentical we’ve fixed this for requests, so let’s fix it for the responses as well.

[isidentical]

Seems like this happen during the adapter, so we should be able to simply build our own HTTPAdapter and use the actual values from the urllib3's HeaderDict. Will send a patch!