AxisArray broadcast container type

Currently AxisArrays do not use the container type information from the parent when working with broadcast. Typically this is fine but in some cases this can raise an exception:

julia> using DataArrays, AxisArrays

julia> a = AxisArray(@data([1, NA, 3]), 'a':'c')
1-dimensional AxisArray{Int64,1,...} with axes:
    :row, 'a':1:'c'
And data, a 3-element DataArrays.DataArray{Int64,1}:
 1
  NA
 3

julia> parent(a) .== 1
3-element DataArrays.DataArray{Bool,1}:
  true
      NA
 false

julia> a .== 1
ERROR: MethodError: Cannot `convert` an object of type DataArrays.NAtype to an object of type Bool
This may have arisen from a call to the constructor Bool(...),
since type constructors fall back to convert methods.
Stacktrace:
 [1] setindex!(::Array{Bool,1}, ::DataArrays.NAtype, ::Int64) at ./array.jl:549
 [2] macro expansion at ./broadcast.jl:180 [inlined]
 [3] macro expansion at ./simdloop.jl:73 [inlined]
 [4] macro expansion at ./broadcast.jl:174 [inlined]
 [5] _broadcast!(::##1#2, ::BitArray{1}, ::Tuple{Tuple{Bool}}, ::Tuple{Tuple{Int64}}, ::AxisArrays.AxisArray{Int64,1,DataArrays.DataArray{Int64,1},Tuple{AxisArrays.Axis{:row,StepRange{Char,Int64}}}}, ::Tuple{}, ::Type{Val{0}}, ::CartesianRange{CartesianIndex{1}}) at ./broadcast.jl:162
 [6] broadcast_t(::Function, ::Type{Bool}, ::Tuple{Base.OneTo{Int64}}, ::CartesianRange{CartesianIndex{1}}, ::AxisArrays.AxisArray{Int64,1,DataArrays.DataArray{Int64,1},Tuple{AxisArrays.Axis{:row,StepRange{Char,Int64}}}}) at ./broadcast.jl:279
 [7] broadcast_c at ./broadcast.jl:314 [inlined]
 [8] broadcast(::Function, ::AxisArrays.AxisArray{Int64,1,DataArrays.DataArray{Int64,1},Tuple{AxisArrays.Axis{:row,StepRange{Char,Int64}}}}) at ./broadcast.jl:434

Start by running the supplied Julia reproduction with DataArrays and AxisArrays, then trace how AxisArray values are handled during broadcast and how parent container type information is selected. Done means the shown a .== 1 operation no longer raises the NAtype-to-Bool exception and preserves the appropriate parent-container behavior.