Ensure tests pass no matter version of DataFrames we're using

Author: quinnj

test/datastreams.jl

@@ -6,6 +6,10 @@ if typeof(DF[:hiredate]) <: NullableVector
     DF[:hiredate] = NullableArray(Date[isnull(x) ? Date() : Date(get(x)) for x in DF[:hiredate]], [isnull(x) for x in DF[:hiredate]])
     DF[:lastclockin] = NullableArray(DateTime[isnull(x) ? DateTime() : DateTime(get(x)) for x in DF[:lastclockin]], [isnull(x) for x in DF[:lastclockin]])
 else
+    for i = 1:5
+        T = eltype(DF.columns[i])
+        DF.columns[i] = NullableArray(T[isna(x) ? (T <: String ? "" : zero(T)) : x for x in DF.columns[i]], [isna(x) for x in DF.columns[i]])
+    end
     DF.columns[6] = NullableArray(Date[isna(x) ? Date() : Date(x) for x in DF[:hiredate]], [isna(x) for x in DF[:hiredate]])
     DF.columns[7] = NullableArray(DateTime[isna(x) ? DateTime() : DateTime(x) for x in DF[:lastclockin]], [isna(x) for x in DF[:lastclockin]])
 end