Fixed deprecation warning in scrapy.core.engine (#5589)

* Change `download` function logic

* Fix CI error in 3.7 checks

* Make `spider` parameter optional in `_download` function, assign spider value from self if `None`

scrapy/core/engine.py

@@ -257,9 +257,7 @@ class ExecutionEngine:
 
     def download(self, request: Request, spider: Optional[Spider] = None) -> Deferred:
         """Return a Deferred which fires with a Response as result, only downloader middlewares are applied"""
-        if spider is None:
-            spider = self.spider
-        else:
+        if spider is not None:
             warnings.warn(
                 "Passing a 'spider' argument to ExecutionEngine.download is deprecated",
                 category=ScrapyDeprecationWarning,
@@ -267,7 +265,7 @@ class ExecutionEngine:
             )
             if spider is not self.spider:
                 logger.warning("The spider '%s' does not match the open spider", spider.name)
-        if spider is None:
+        if self.spider is None:
             raise RuntimeError(f"No open spider to crawl: {request}")
         return self._download(request, spider).addBoth(self._downloaded, request, spider)
 
@@ -278,11 +276,14 @@ class ExecutionEngine:
         self.slot.remove_request(request)
         return self.download(result, spider) if isinstance(result, Request) else result
 
-    def _download(self, request: Request, spider: Spider) -> Deferred:
+    def _download(self, request: Request, spider: Optional[Spider]) -> Deferred:
         assert self.slot is not None  # typing
 
         self.slot.add_request(request)
 
+        if spider is None:
+            spider = self.spider
+
         def _on_success(result: Union[Response, Request]) -> Union[Response, Request]:
             if not isinstance(result, (Response, Request)):
                 raise TypeError(f"Incorrect type: expected Response or Request, got {type(result)}: {result!r}")