1207 Allow numeric predicates when filtering arrays

[michaelhkay]

Fix #1207

Also a minor change: $V[23, "fred"] now throws FORG0006 rather than XPTY0004. This keeps it compatible with 3.1 (in case anyone is catching the errors), and is more uniform: it seems unreasonable for $V[23, "fred"] and $V["fred", 23] to throw different errors.