Use Undef markers for undefined fields

Previously BSON would skip these entirely causing mis-ordered
`setfield!` calls when loading the struct.

src/extensions.jl

@@ -108,8 +108,9 @@ tags[:array] = d ->
 
 # Structs
 
+struct Undef end
 structdata(x) = isprimitivetype(typeof(x)) ? reinterpret_(UInt8, [x]) :
-    Any[getfield(x,f) for f in fieldnames(typeof(x)) if isdefined(x, f)]
+    Any[isdefined(x, f) ? getfield(x,f) : Undef() for f in fieldnames(typeof(x))]
 
 function lower(x)
   BSONDict(:tag => "struct", :type => typeof(x), :data => structdata(x))
@@ -119,6 +120,7 @@ initstruct(T) = ccall(:jl_new_struct_uninit, Any, (Any,), T)
 
 function newstruct!(x, fs...)
   for (i, f) = enumerate(fs)
+    isa(f, Undef) && continue
     f = convert(fieldtype(typeof(x),i), f)
     ccall(:jl_set_nth_field, Nothing, (Any, Csize_t, Any), x, i-1, f)
   end