Revert "Avoid interpreting :a in :a=>1 as a column vector"

Reverts JuliaStats/DataFramesMeta.jl#10

@tonyhffong, I couldn't get this to work. Here is an example that doesn't work for me:

```julia
d = DataFrame(a = 1:10, b = rand(10))
@with(d, [:a => :a, :b => 2 * :b])
```