SessionManager.fetch() pops `method` from `_session_kwargs`,
which mutates the original request dict. When the engine retries
a blocked request via request.copy(), the copy no longer has
`method`, so it defaults to GET.
Steps to reproduce:
1. Yield Request(url, method="POST", data=...)
2. Target returns a response that triggers is_blocked()
3. Engine retries via request.copy() → second fetch uses GET
Fix: copy the kwargs dict before popping, so the original
request stays intact.
karesansui