Ensure XML.attributes(node) === nothing when node has no attributes

[TimG1964]

Speculative suggestion to fix issue #33 - proposed for review.
Tests pass locally but I think this needs deeper understanding than I can bring.
I have focused only on XML.Elements.

[TimG1964]

I added another change to setindex() to deal with the case when XML.attributes === nothing

[TimG1964]

I think I'm going to need more help here.

What I really want to do around line 208 is

    o.attributes = isnothing(o.attributes) ? OrderedDict{String, String}() : o.attributes

Except, of course, a node is immutable, so this doesn't work.

What I've suggested in my last change doesn't work because it simply assigns o to a new Node and so it no longer updates the original node o in place.

I think I can make the change from nothing to OrderedDict in my code before the call to setindex!(), but that isn't really a solution, is it?.

Help!