net/http: send Content-Range if no byte range overlaps
RFC 7233, section 4.4 says:
>>>
For byte ranges, failing to overlap the current extent means that the
first-byte-pos of all of the byte-range-spec values were greater than the
current length of the selected representation. When this status code is
generated in response to a byte-range request, the sender SHOULD generate a
Content-Range header field specifying the current length of the selected
representation
<<<
Thus, we should send the Content-Range only if none of the ranges
overlap.