mem_hdr::hasContigousContentRange() should return true for empty ranges

mem_hdr::hasContigousContentRange() should return true for empty ranges

mem_hdr::hasContigousContentRange() is called exclusively from
StoreEntry::mayStartSwapOut() via memObject::isContiguous(). In theory, that
mayStartSwapOut() may happen when we have not written anything to mem_hdr yet
(not even HTTP response headers). In that case, mayStartSwapout() should not
refuse to swap the entry out on non-contiguous grounds, but it was doing that
because hasContigousContentRange() mishandled the empty range case.

XXX: Calling hasContigousContentRange() [just] from mayStartSwapout() is just
wrong because what may be contiguous now may become fragmented later.