OrderedDict for dictrowtable (#277)

* `OrderedDict` for `dictrowtable`

* Fixes docstrings on `dictcolumntable` and `dictrowtable`

* Test of `size` implementation on `GenericColumn` with Julia nightly

* Fallback from test in previous commit

src/dicts.jl

@@ -7,7 +7,7 @@ end
     Tables.dictcolumntable(x) => Tables.DictColumnTable
 
 Take any Tables.jl-compatible source `x` and return a `DictColumnTable`, which
-can be thought of as a `Dict` mapping column names as `Symbol`s to `AbstractVector`s.
+can be thought of as a `OrderedDict` mapping column names as `Symbol`s to `AbstractVector`s.
 The order of the input table columns is preserved via the `Tables.schema(::DictColumnTable)`.
 
 For "schema-less" input tables, `dictcolumntable` employs a "column unioning" behavior,
@@ -126,7 +126,7 @@ end
     Tables.dictrowtable(x) => Tables.DictRowTable
 
 Take any Tables.jl-compatible source `x` and return a `DictRowTable`, which
-can be thought of as a `Vector` of `Dict` rows mapping column names as `Symbol`s to values.
+can be thought of as a `Vector` of `OrderedDict` rows mapping column names as `Symbol`s to values.
 The order of the input table columns is preserved via the `Tables.schema(::DictRowTable)`.
 
 For "schema-less" input tables, `dictrowtable` employs a "column unioning" behavior,
@@ -141,12 +141,12 @@ the union behavior is needed.
 function dictrowtable(x)
     names = Symbol[]
     seen = Set{Symbol}()
-    types = Dict{Symbol, Type}()
+    types = OrderedDict{Symbol, Type}()
     r = rows(x)
     L = Base.IteratorSize(typeof(r))
-    out = Vector{Dict{Symbol, Any}}(undef, Base.haslength(r) ? length(r) : 0)
+    out = Vector{OrderedDict{Symbol, Any}}(undef, Base.haslength(r) ? length(r) : 0)
     for (i, drow) in enumerate(r)
-        row = Dict{Symbol, Any}(nm => getcolumn(drow, nm) for nm in columnnames(drow))
+        row = OrderedDict{Symbol, Any}(nm => getcolumn(drow, nm) for nm in columnnames(drow))
         add!(row, 0, :_, out, L, i)
         if isempty(names)
             for (k, v) in row