Allow docs missing a root element to be created and printed. Fixes #121.

[sjp]

This fixes the issue described in #121.

This is mostly achieved by checking whether a document has a root element before attempting to print it.

One other minor bugfix is that an xml_document object will now print its children if they were previously absent. An example:

Before:

> library(xml2)
> doc <- xml_new_document()
> root <- xml_add_child(doc, "root")
> doc
{xml_document}

After:

> library(xml2)
> doc <- xml_new_document()
> root <- xml_add_child(doc, "root")
> doc
{xml_document}
<root>

[jimhester]

This looks good, only thing needed is a short note to NEWS.md explaining the fix and thanking yourself.

[jimhester]

Don't worry about the NEWS, I will add it manually.

[sjp]

Great, thanks!