Invalid combinators to raise ArgumentError

[bagp1]

Based on #1396

The reporter encountered two issues:

1. Passing a combinator value as a symbol doesn't match the expected combinator.
2. Invalid arguments silently misbehavior rather than raise argument errors even when the dangerous method #ransack! is called.

I can see why unrecognised combinators default to "AND", it's the more conservative, safe option. But it's very surprising and a potential source of bugs for downstream users. Also the users have intentionally called the dangerous method, so should be error handling as appropriate for their domain. The right thing to do is to raise on invalid args.

However we'd best minimize impact for users currently passing invalid args. The most obvious errors would be rather than passing the expected "or", they might be passing one of "OR", :or and not noticing the fault. We can correct behavior in these common cases by downcasing and converting to a string. Indeed Ransack already does conversion to a string in some pathways, just not as part of a search. Also Anything more exotic is really unrecognisable and an error.

This PR comes with a set of tests describing expected behavior and including a few that would have failed prior to the changes.