Slice filter: clear original Accept-Ranges.

Slice filter: clear original Accept-Ranges.

The slice filter allows ranges for the response by setting the r->allow_ranges
flag, which enables the range filter.  If the range was not requested, the
range filter adds an Accept-Ranges header to the response to signal the
support for ranges.

Previously, if an Accept-Ranges header was already present in the first slice
response, client received two copies of this header.  Now, the slice filter
removes the Accept-Ranges header from the response prior to setting the
r->allow_ranges flag.