Add method for getting order using customer_id with tests

Author: Paulinakhew

shopify/resources/order.py

@@ -4,6 +4,15 @@
 
 
 class Order(ShopifyResource, mixins.Metafields, mixins.Events):
+    _prefix_source = "/customers/$customer_id/"
+
+    @classmethod
+    def _prefix(cls, options={}):
+        customer_id = options.get("customer_id")
+        if customer_id:
+            return "%s/customers/%s" % (cls.site, customer_id)
+        else:
+            return cls.site
 
     def close(self):
         self._load_attributes_from_response(self.post("close"))