fix collect(DataFrameRow)

I think `collect(::DataFrameRow)` was broken:

```julia
julia> df = DataFrame(A = [1,1,1,1], B = [2,2,2,2], C = [3,3,3,3])
4x3 DataFrame
|-----|---|---|---|
| Row | A | B | C |
| 1   | 1 | 2 | 3 |
| 2   | 1 | 2 | 3 |
| 3   | 1 | 2 | 3 |
| 4   | 1 | 2 | 3 |

julia> for r in eachrow(df)
         collect(r)
       end
ERROR: `convert` has no method matching convert(::Type{String}, ::Symbol)
 in convert at base.jl:21
 in setindex! at array.jl:307
 in collect at /Users/westley/.julia/v0.3/DataFrames/src/dataframerow/dataframerow.jl:29
 in anonymous at no file:2
```

Maybe using `String` made sense in certain cases? In that case perhaps we need to chance the tuple returned to be `(Any, Any)`