fix problem with NoneType response headers

Author: vladkhard

openprocurement_client/clients.py

@@ -88,10 +88,11 @@ def _get_resource_item(self, url, headers=None):
         response_item = self.request('GET', url, headers=_headers)
         if response_item.status_code == 200:
             data = loads(response_item.text)
-            if "x-revision-n" in response_item.headers:
-                data["x_revision_n"] = response_item.headers["x-revision-n"]
-            if "x-revision-data" in response_item.headers:
-                data["x_revision_data"] = response_item.headers["x-revision-data"]
+            if isinstance(response_item.headers, dict):
+                if "x-revision-n" in response_item.headers:
+                    data["x_revision_n"] = response_item.headers["x-revision-n"]
+                if "x-revision-data" in response_item.headers:
+                    data["x_revision_data"] = response_item.headers["x-revision-data"]
             return munchify(data)
         raise InvalidResponse(response_item)