Requests with empty body not None missing header content-Length

[hxzhao527]

I read the historical issue #2240, #3654 and the fix #3630, but it eventually was reverted by #3633 because of curl's ci failed #13380.

As @paolobarbolini showed in reqwest issue #2240, curl will send content-length when body is specified even if the body is empty.

So I wonder if we can make a distinction between no body and empty body. Set content-length to 0 when the body is empty. @seanmonstar