Error when filtering by Union{Bool, Null}

On Julia v0.6, using DataFrames, DataValues and Query masters:

julia> using Query, DataFrames

julia> df = DataFrame(a=[1,2,3], b=Union{Null,Bool}[true, true, false], c=[true, true, false])
3×3 DataFrames.DataFrame
│ Row │ a │ b     │ c     │
├─────┼───┼───────┼───────┤
│ 1   │ 1 │ true  │ true  │
│ 2   │ 2 │ true  │ true  │
│ 3   │ 3 │ false │ false │

julia> df|>@where(_.c)|>DataFrame  # 1. filtering by non-nullable, OK
2×3 DataFrames.DataFrame
│ Row │ a │ b    │ c    │
├─────┼───┼──────┼──────┤
│ 1   │ 1 │ true │ true │
│ 2   │ 2 │ true │ true │

julia> df|>@where(_.b)|>DataFrame # 2. filtering by nullable, FAILS
ERROR: TypeError: non-boolean (DataValues.DataValue{Bool}) used in boolean context
Stacktrace:
 [1] start(::QueryOperators.EnumerableWhere{NamedTuples._NT_a_b_c{Int64,DataValues.DataValue{Bool},Bool},QueryOperators.EnumerableIterable{NamedTuples._NT_a_b_c{Int64,DataValues.DataValue{Bool},Bool},IterableTables.DataFrameIterator{NamedTuples._NT_a_b_c{Int64,DataValues.DataValue{Bool},Bool},Tuple{Array{Int64,1},Array{Union{Bool, Nulls.Null},1},Array{Bool,1}}}},##14#16}) at /home/astukalov/.julia/v0.6/QueryOperators/src/enumerable/enumerable_where.jl:33
 [2] macro expansion at /home/astukalov/.julia/v0.6/IterableTables/src/integrations/dataframes-null.jl:91 [inlined]
....

julia> df|>@where(_.b==true)|>DataFrame # 3. comparison of nullable with the value, OK
2×3 DataFrames.DataFrame
│ Row │ a │ b    │ c    │
├─────┼───┼──────┼──────┤
│ 1   │ 1 │ true │ true │
│ 2   │ 2 │ true │ true │

I don't know how to properly fix the second test (I assume it should have the same behaviour as the 1st), e.g. should Base.convert(::Type{Bool}, ::DataValue{Bool}) be defined or filtering lambda generation tweaked?

Reproduce the nullable and non-nullable filtering examples on the Julia v0.6 setup, then inspect QueryOperators/src/enumerable/enumerable_where.jl and IterableTables/src/integrations/dataframes-null.jl at the stack-trace locations. Determine how nullable boolean predicates should behave and add coverage so filtering by Union{Bool, Null} has the intended consistent result.