Fix single node

[BenedictBoyle]

Ran into a few issues when using this very handy module:

1. When dumping very simple dicts like {'a', 'b'} you get an error, in fact there's no way to get an output of <a>b</a> even using _value keys.
2. When dumping a dict, the code strips the _attrs keys from the underlying dict, so dumping the same dict more than once gives different results (no attributes on subsequent dumps).

These minor amendments should address both of these issues - I've included test cases for both.

Many thanks.