fix(kv): use json multipart syntax

Author: RobertCraigie

src/resources/kv/namespaces/values.ts

@@ -37,7 +37,12 @@ export class Values extends APIResource {
     return (
       this._client.put(
         `/accounts/${account_id}/storage/kv/namespaces/${namespaceId}/values/${keyName}`,
-        Core.multipartFormRequestOptions({ query: { expiration, expiration_ttl }, body, ...options }),
+        Core.multipartFormRequestOptions({
+          query: { expiration, expiration_ttl },
+          body,
+          __multipartSyntax: 'json',
+          ...options,
+        }),
       ) as Core.APIPromise<{ result: ValueUpdateResponse | null }>
     )._thenUnwrap((obj) => obj.result);
   }