find() can not find tag when it is only-one

[yikeshu0611]

(html <- span("bar"))
tagQ <- tagQuery(html)
tagQ$find('span')

Result:

`$allTags()`:
<span>bar</span>

`$selectedTags()`: (Empty selection)

html is only one tag of span. So if I use find() to locate span tag, the result should be it self.

[cpsievert]

Thanks. We're aware of the confusion, and will likely fix this behavior. Closing here since this is essentially a duplicate of #334