souphttpsrc: Should send "Range" when offset is 0

Submitted by Paul Kim

Link to original bug (#792664)

Description

Cannot seek contents in a server and the reason is that the server sents different file headers when "Range" is included in request header or not.
For this reason, only gstreamer cannot do seek, but ffmpeg in chromium covers this case. To improve stability of gstreamer, should send "Range: 0-" for all cases.