Adds tests for indexed sort.

Currently the ruby ones don't really work and are commented out.
Probably going to need a hand from mlucy to fix those.
Adds docs for indexed order_by.
Fixes the ruby driver's order_by optarg parsing.

The problem was that optargs were being parsed out after rewrites were
applied. These rewrites changed order_by to orderby so the optarg
parsing missed it.

Also this turns on tests that were failing due to this problem.
Adds in a test for truncated keys.

Also fixes a bug it uncovered.
Adds an order_by with a sindex that errors.