Used let block; improves prior version of fix
Used let block; improves prior version of fix

This implements remark by  @johnmyleswhite; my concern was of variable interception
elsewhere in the module, but this is solved by the let block.

tkelman/DataFrames.jl,pwoolcoc/DataFrames.jl,ccphillippi/DataFrames.jl,anriseth/DataFrames.jl,zhmz90/DataFrames.jl,emilbayes/DataFrames.jl,campanja-forks/DataFrames.jl,niloynibhochaudhury/DataFrames.jl,JuliaPackageMirrors/DataFrames.jl,tshort/DataFrames.jl,scls19fr/DataFrames.jl,armgong/DataFrames.jl,JuliaStats/DataFrames.jl