toString() emits `DocumentFragment`s as <undefined> tags

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When I add DocumentFragments to a node, the node gets rendered with <undefined> tags.

Sample code:

var document = require('min-document')
var fragment = document.createDocumentFragment()
fragment.appendChild(document.createTextNode('hello'))
var span = document.createElement('span')
span.appendChild(fragment)
span.toString()

Output:

"<span><undefined>hello</undefined></span>"