[ruby] fix: HEAD requests (#9436)

When WEBrick receives `HEAD` requests it discards the body (i.e.
`req.body.nil? => true`), this causes Vercel to throw a
`BODY_NOT_A_STRING_FROM_FUNCTION` since it is expecting the serverless
function to respond with a string in the body.

---------

Co-authored-by: Nathan Rajlich <n@n8.io>
Co-authored-by: Steven <steven@ceriously.com>

packages/ruby/test/fixtures/01-cowsay/now.json

@@ -1,5 +1,8 @@
 {
   "version": 2,
   "builds": [{ "src": "index.rb", "use": "@vercel/ruby" }],
-  "probes": [{ "path": "/", "mustContain": "gem:RANDOMNESS_PLACEHOLDER" }]
+  "probes": [
+    { "path": "/", "mustContain": "gem:RANDOMNESS_PLACEHOLDER" },
+    { "path": "/", "method": "HEAD", "status": 200 }
+  ]
 }

packages/ruby/vc_init.rb

@@ -73,7 +73,7 @@ def webrick_handler(httpMethod, path, body, headers)
   {
     :statusCode => res.code.to_i,
     :headers => res_headers,
-    :body => res.body,
+    :body => res.body.nil? ? "" : res.body,
   }
 end
 

test/lib/deployment/test-deployment.js

@@ -384,7 +384,7 @@ async function fetchDeploymentUrl(url, opts) {
   for (let i = 0; i < 50; i += 1) {
     const resp = await fetch(url, opts);
     const text = await resp.text();
-    if (text && !text.includes('Join Free')) {
+    if (typeof text !== 'undefined' && !text.includes('Join Free')) {
       return { resp, text };
     }