readdlm: handle white space after last column properly

ref: discussion in #5856
Also fix undef columns for empty lines within rows.

Now:
````
julia> readdlm(IOBuffer("\"test1\" \"test2\" \n\"cuac1\" \"cuac2\""))
2x2 Array{Any,2}:
 "test1"  "test2"
 "cuac1"  "cuac2"

julia> readdlm(IOBuffer(" \n\n\n"), ' ')
3x2 Array{Any,2}:
 ""  ""
 ""  ""
 ""  ""
````