fix ref/value problem in unique for pooled data

There is an error in the `unique` function for `PooledDataArray`: when storing unique values, it tries to push the ref to the `unique_values` array which is declared with the same type as pooled data.  This results in a call to `convert` in the base array code which throws an error (see https://github.com/JuliaStats/DataFrames.jl/issues/627).