Set content-type header to text/plain

This change ensures content-type header is set to ensure correect
encoding detection, eg: on pwsh.

Resolves: python-poetry/poetry#4845

vercel.json

@@ -1,4 +1,24 @@
 {
+  "headers": [
+    {
+      "headers": [
+        {
+          "key": "Content-Type",
+          "value": "text/plain; charset=utf-8"
+        }
+      ],
+      "source": "/"
+    },
+    {
+      "headers": [
+        {
+          "key": "Content-Type",
+          "value": "text/plain; charset=utf-8"
+        }
+      ],
+      "source": "/(.+)\\.py"
+    }
+  ],
   "rewrites": [
     {
       "destination": "/install-poetry.py",