Fix header processing (#226)

* Scrapy headers were being updated for every Playwright request,
  they should only be updated for the Playwright request that
  corresponds to the original Scrapy one
* Referer header from the Playwright request was not added to the
  output of scrapy_playwright.headers.use_scrapy_headers

scrapy_playwright/handler.py

@@ -494,13 +494,8 @@ async def _request_handler(route: Route, playwright_request: PlaywrightRequest)
                         self.browser_type_name, playwright_request, headers
                     )
                 )
-            # the request that reaches the callback should contain the final headers
-            headers.clear()
-            headers.update(final_headers)
-            del final_headers
 
-            # if the request is triggered by scrapy, not playwright
-            original_playwright_method: str = playwright_request.method
+            # if the current request corresponds to the original scrapy one
             if (
                 playwright_request.url.rstrip("/") == url.rstrip("/")
                 and playwright_request.is_navigation_request()
@@ -509,7 +504,13 @@ async def _request_handler(route: Route, playwright_request: PlaywrightRequest)
                     overrides["method"] = method
                 if body:
                     overrides["post_data"] = body.decode(encoding)
+                # the request that reaches the callback should contain the final headers
+                headers.clear()
+                headers.update(final_headers)
 
+            del final_headers
+
+            original_playwright_method: str = playwright_request.method
             try:
                 await route.continue_(**overrides)
                 if overrides.get("method"):

scrapy_playwright/headers.py

@@ -23,9 +23,14 @@ async def use_scrapy_headers(
     scrapy_headers_str.setdefault("user-agent", playwright_headers.get("user-agent"))
 
     if playwright_request.is_navigation_request():
+        # if referer header is set via playwright_page_goto_kwargs
+        if referer := playwright_headers.get("referer"):
+            scrapy_headers_str.setdefault("referer", referer)
+
+        # otherwise it fails with playwright.helper.Error: NS_ERROR_NET_RESET
         if browser_type == "firefox":
-            # otherwise this fails with playwright.helper.Error: NS_ERROR_NET_RESET
             scrapy_headers_str["host"] = urlparse(playwright_request.url).netloc
+
         return scrapy_headers_str
 
     # override user agent, for consistency with other requests