fix incorrect rsearchindex with utf8 query

Disclaimer: I don't know much about UTF8, so am not sure it's the good fix.

In `rsearchindex(s::ByteString, t::ByteString, i::Integer)`, the call to
`rsearchindex` on the underlying data must account for the possibility
that `s[i]` is a non-ASCII character, i.e. `i` must be augmented to
reference the last byte of `s[i]`.