Ensure raw/flat header logic always uses strings (Node allows numbers)

This doesn't matter for our own logic, since we should always be
handling numbers, but when reading state from requests built elsewhere
(from 'ws' internals) we can see numbers sometimes.

Author: pimterry

src/util/header-utils.ts

@@ -127,9 +127,9 @@ export function objectHeadersToRaw(headers: Headers): RawHeaders {
         if (value === undefined) continue; // Drop undefined header values
 
         if (Array.isArray(value)) {
-            value.forEach((v) => rawHeaders.push([key, v]));
+            value.forEach((v) => rawHeaders.push([key, v.toString()]));
         } else {
-            rawHeaders.push([key, value]);
+            rawHeaders.push([key, value.toString()]);
         }
     }
 
@@ -147,11 +147,11 @@ export function objectHeadersToFlat(headers: Headers): string[] {
         if (Array.isArray(value)) {
             value.forEach((v) => {
                 flatHeaders.push(key);
-                flatHeaders.push(v);
+                flatHeaders.push(v.toString());
             });
         } else {
             flatHeaders.push(key);
-            flatHeaders.push(value);
+            flatHeaders.push(value.toString());
         }
     }