text: Title: Different behavior of `get_header` between urllib.Request and WrappedRequest
  Body: <!--
  
  Thanks for taking an interest in Scrapy!
  
  If you have a question that starts with "How to...", please see the Scrapy Community page: https://scrapy.org/community/.
  The GitHub issue tracker's purpose is to deal with bug reports and feature requests for the project itself.
  
  Keep in mind that by filing an issue, you are expected to comply with Scrapy's Code of Conduct, including treating everyone with respect: https://github.com/scrapy/scrapy/blob/master/CODE_OF_CONDUCT.md
  
  The following is a suggested template to structure your issue, you can find more guidelines at https://doc.scrapy.org/en/latest/contributing.html#reporting-bugs
  
  -->
  
  ### Description
  
  I believe `WrappedRequest` is a class for supporting methods in `urllib.Request`, but I found a behavior of `get_header` method in `WrappedRequest` is different from `urllib.Requests`.
  When trying to get a header which is not present without default value, it should return `None` value (https://docs.python.org/3/library/urllib.request.html#urllib.request.Request.get_header), 
  but `get_header` in `WrappedRequest` raises `TypeError: to_unicode must receive a bytes or str object, got NoneType`.
  
  ### Steps to Reproduce
  
  ```
  from urllib.request import Request as _Request
  from scrapy.http.request import Request
  from scrapy.http.cookies import WrappedRequest
  
  a = _Request(url="https://a.example")
  print(a.get_header('xxxx'))
  
  b = WrappedRequest(Request(url="https://a.example"))
  print(b.get_header('xxxx'))
  ```
  
  **Expected behavior:** 
  
  ```
  None
  None
  ```
  
  **Actual behavior:**
  
  ```
  None
  TypeError: to_unicode must receive a bytes or str object, got NoneType
  ```
  
  ### Additional context
  
  This issue is currently not a problem when interacting with the `CookieJar` class.
  Thus, it is not an urgent matter unless one is importing and using `WrappedRequest`. 
  However, I think it would enhance the reliability in this project.
  Thank you!