chore(internal/client): fix form-urlencoded requests

Author: stainless-app[bot]

src/client.ts

@@ -854,6 +854,14 @@ export class Kernel {
         (Symbol.iterator in body && 'next' in body && typeof body.next === 'function'))
     ) {
       return { bodyHeaders: undefined, body: Shims.ReadableStreamFrom(body as AsyncIterable<Uint8Array>) };
+    } else if (
+      typeof body === 'object' &&
+      headers.values.get('content-type') === 'application/x-www-form-urlencoded'
+    ) {
+      return {
+        bodyHeaders: { 'content-type': 'application/x-www-form-urlencoded' },
+        body: this.stringifyQuery(body as Record<string, unknown>),
+      };
     } else {
       return this.#encoder({ body, headers });
     }