Fix MethodError when calling groupby, unstack, etc (#274)

* Fix MethodError when calling groupby, unstack, etc

* Fix PooledDataArray for types such that convert(typeof(d), unique(d)) does not exist.
Add 2 unit tests for issue #273

* More concise notation for the convert in PDAs

Author: LouisMagarshack

src/pooleddataarray.jl

@@ -137,7 +137,7 @@ end
 function PooledDataArray{T,R<:Integer,N}(d::AbstractArray{T, N},
                                          m::AbstractArray{Bool, N},
                                          r::Type{R} = DEFAULT_POOLED_REF_TYPE)
-    pool = convert(Array, unique(d[.!m]))
+    pool = convert(Vector{T}, unique(d[.!m]))
     if method_exists(isless, (T, T))
         sort!(pool)
     end

test/pooleddataarray.jl

@@ -140,4 +140,15 @@
     end
     z = PooledDataArray([1, 2], [1, 2, 3])
     @test sort(unique(z)) == DataArray([1, 2])
+
+
+    # Issue #273
+    #   Ensure that PooledDataArray works for arrays with non leaf type elements.
+    #   Those two examples used to throw exceptions.
+    x = AbstractString["a"]
+    m = Bool[false]
+    r = UInt32
+    @test isa(PooledDataArray(x, m, r), DataArrays.PooledDataArray{AbstractString, UInt32, 1})
+    x = Integer[1]
+    @test isa(PooledDataArray(x, m, r), DataArrays.PooledDataArray{Integer, UInt32, 1})
 end