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

When I pass a `Dict{Symbol,Any}` to a function in this framework, I want to avoid the symbol keys being interpreted as a column vector.
```julia
@with(df, mycolfunc( :y, hints = Dict{Symbol,Any}( :a => 1, :b => "" ) )
```
This PR proposes to bypass the key symbol replacements under `:(=>)` expression head.