text: Title: Spider.logger not logging custom extra information
  Body: I noticed the implicit behavior of the Spider.logger: when logging with extra, extra ultimately do not end up in the log because they are overwritten by default `process` method of [LoggerAdapter](https://github.com/scrapy/scrapy/blob/master/scrapy/spiders/__init__.py#L47)
  
  Current logic:
  ```py
  >>> self.logger.info("test log", extra={"test": "very important information"})
  {"message": "test log", "spider": "spider_name"}
  ```
  
  
  
  Expected logic:
  ```py
  >>> self.logger.info("test log", extra={"test": "very important information"})
  {"message": "test log", "spider": "spider_name", "test": "very important information"}
  ```
  
  