Fix bug that invalid range error when requesting range starting 0

Since the change b782a56b, we use endOffsetOverride_ as the return
value of getEndByte(). But aria2 does not send Range header field when
range starts 0 (this is because some server returns error if it
received Range: 0-), and the HttpRequest::isRangeSatisfied() checks
the equality of getEndByte() and the end byte in response header and
fails. The fix is send Range header if getEndByte() is set.