handle trailing whitespace in whitespace separated data

Fixes the condition where additional white spaces in the last line of a whitespace delimited file threw errors. They will now be ignored.

````
julia> readdlm(IOBuffer("1 2 \n3 4 \n"))
2x2 Array{Float64,2}:
 1.0  2.0
 3.0  4.0

julia> readdlm(IOBuffer("1 2\n3 4 \n"))
2x2 Array{Float64,2}:
 1.0  2.0
 3.0  4.0
````