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
```