Fix find() when called with function and non-StridedArray

The previous code expected testfun to be vectorized, which does not match
the behavior of other methods, nor the documentation. Especially visible
when passing a sparse matrix. Always return an Array rather than an object
similar to the input, because it makes no sense to return a sparse vector.
Remove the method matching any object so that an error is raised rather
than returning incorrect results on non-AbstractArray (e.g. Set).


This fixes the bug reported by i.pallikarakis-11 on the mailing list, namely:
```julia
A=speye(Bool,10)
find(x->x==true,A)
->0-element Array{Int64,1}

find(x->x==true,full(A))
->10-element Array{Int64,1}:
   1
  12
  23
  34
  45
  56
  67
  78
  89
 100
```

I'm not completely sure about the `AbstractArray` requirement, but it doesn't make sense to me to return *indexes* for non-arrays. The previous code would happily work on `Set`s, but would return nonsensical results.

(BTW, I'm aware that calling `find(testf, x)` with `x` a sparse matrix is not very efficient, but at least it must not give wrong results.)
Yeah, not pretty. :-/ I could have fixed that. Worth another commit?