replace "filesize - 1" trick in http with proper 416 handling

Our http client requests the "filesize - 1" for the small edgecase of
handling a file which was completely downloaded, but not yet moved to
the correct place as we get 416 errors in that case, but as we can
handle 416 returns now we just special-case the situation of requesting
the exact filesize and handle it as a 200 without content instead.