add support for file hosts which forbid request with 1. absent range header, 2. large range header

try to make a request with absent range header at first, if it fails,
simply make another request with range header with small value
like "Range: bytes=0-1" to acquire content length from the response.
i notice aria2 tries to split a download into large pieces like 200+mb
however some hosts does not allow that, please consider options like:
"--max-piece-size=1m" (equal to "Range: bytes=x-y"; y=x+1024*1024-1)
to limit how big a piece can be.

known file hosts that behave like this: baidu wangpan

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