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.

Fixes #15798.

Change-Id: Ic9a3e1b3a8730398b4bdff877a8f2fd2e30149e3
Reviewed-on: https://go-review.googlesource.com/24212
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>