fix: isolate MLI header to prevent unintended propagation

Author: rostyk-kanafotskyy

src/endpoints/multi-location-inventories.js

@@ -6,7 +6,14 @@ class MultiLocationInventories {
   constructor(endpoint) {
     const config = { ...endpoint }
 
-    config.headers['ep-inventories-multi-location'] = true
+    Object.defineProperty(config, 'headers', {
+      get() {
+        return {
+          ...endpoint.headers,
+          'ep-inventories-multi-location': true
+        }
+      }
+    })
 
     this.request = new RequestFactory(config)