fix(client): send content-type header for requests with an omitted optional body

Author: stainless-app[bot]

src/client.ts

@@ -1030,11 +1030,19 @@ export class ModernTreasury {
     return () => controller.abort();
   }
 
-  private buildBody({ options: { body, headers: rawHeaders } }: { options: FinalRequestOptions }): {
+  private buildBody({ options }: { options: FinalRequestOptions }): {
     bodyHeaders: HeadersLike;
     body: BodyInit | undefined;
   } {
+    const { body, headers: rawHeaders } = options;
     if (!body) {
+      // A resource method always passes a `body` key when its operation defines a
+      // request body, even if the caller omitted an optional body param. Keep the
+      // content-type for those, and only elide it for operations with no body at
+      // all (e.g. GET/DELETE).
+      if (body == null && 'body' in options) {
+        return this.#encoder({ body, headers: buildHeaders([rawHeaders]) });
+      }
       return { bodyHeaders: undefined, body: undefined };
     }
     const headers = buildHeaders([rawHeaders]);