RFC: Fix findin for String Arrays

See https://groups.google.com/forum/#!topic/julia-users/KcqDAr7Sb8Q

```julia
julia> ss = fill("a",2,2)
2x2 ASCIIString Array:
 "a"  "a"
 "a"  "a"

julia> findin(ss, "a")
0-element Int64 Array
```

The problem is that `findin` is defined as 

```julia
function findin(a, b)
    ind = Array(Int, 0)
    bset = Set(b...)
    for i = 1:length(a)
        if has(bset, a[i])
            push!(ind, i)
        end
    end
    ind
end
```

and `bset` becomes a set of characters.