root logger's level is always set to NOTSET

[joelkim]

Currently, the log level of a root logger is always automatically re-set to NOTSET level in install_scrapy_root_handler function call if I add handlers to root logger:

scrapy/scrapy/utils/log.py

Line 110 in 108f8c4

logging.root.setLevel(logging.NOTSET)

Can anybody explain why this level reset is required?
This issue is relevant to issue #2352, #2149, I think.

[thernstig]

@joelkim please see my answer in #2149. I believe this issue can be closed as a clone of that.