Put spaces between multiple attributes in element#to_xml

Addresses the output issue from #32. Code changed, test added.
There's no need for using `map` and `join` here, nor is a conditional assignment needed here (the `unless` applies to the entire assignment, not just the `<<` call). All you need to do is instead of this:

    attrs << attr.to_xml

Use this:

    attrs << " #{attr.to_xml}"