[python] Cope with duplicate header values (#9205)

There are times when a request can arrive for a Python function with headers as a list. One of those examples is this header `x-vercel-proxied-for` which apparently is set twice. Example:

`[b'x-vercel-proxied-for', [b'207.81.134.243', b'172.71.147.74']]`

I took a quick scan through the other Python server implementations and I don't think any of them manipulate the value of the HTTP headers, the way the ASGI one does so I think we are good there.

To reproduce:

`curl https://..../ -H "foo: bar" -H "foo: bar"`

Will fail.

Fixes: https://github.com/vercel/vercel/issues/9132

packages/python/test/fixtures/32-fail-duplicate-header/index.py

@@ -0,0 +1,14 @@
+async def app(scope, receive, send):
+    assert scope["type"] == "http"
+    await send(
+        {
+            "type": "http.response.start",
+            "status": 200,
+        }
+    )
+    await send(
+        {
+            "type": "http.response.body",
+            "body": b"hello world"
+        }
+    )

packages/python/test/fixtures/32-fail-duplicate-header/probe.js

@@ -0,0 +1,18 @@
+const execa = require('execa');
+
+module.exports = async function ({ deploymentUrl, fetch }) {
+  const probeUrl = `https://${deploymentUrl}`;
+  const result = await execa('curl', [
+    probeUrl,
+    '-s',
+    '-H',
+    'foo: bar',
+    '-H',
+    'foo: bar',
+  ]);
+  if (result.stdout.includes('FUNCTION_INVOCATION_FAILED')) {
+    throw new Error(
+      'Duplicate headers should not cause a function invocation failure'
+    );
+  }
+};

packages/python/test/fixtures/32-fail-duplicate-header/vercel.json

@@ -0,0 +1,9 @@
+{
+  "version": 2,
+  "builds": [
+    {
+      "src": "*.py",
+      "use": "@vercel/python"
+    }
+  ]
+}

packages/python/vc_init.py

@@ -276,6 +276,14 @@ elif 'app' in __vc_variables:
             query = url.query.encode()
             path = url.path
 
+            headers_encoded = []
+            for k, v in headers.items():
+                # Cope with repeated headers in the encoding.
+                if isinstance(v, list):
+                    headers_encoded.append([k.lower().encode(), [i.encode() for i in v]])
+                else:
+                    headers_encoded.append([k.lower().encode(), v.encode()])
+
             scope = {
                 'server': (headers.get('host', 'lambda'), headers.get('x-forwarded-port', 80)),
                 'client': (headers.get(
@@ -285,7 +293,7 @@ elif 'app' in __vc_variables:
                 'scheme': headers.get('x-forwarded-proto', 'http'),
                 'root_path': '',
                 'query_string': query,
-                'headers': [[k.lower().encode(), v.encode()] for k, v in headers.items()],
+                'headers': headers_encoded,
                 'type': 'http',
                 'http_version': '1.1',
                 'method': payload['method'],