Name from content without descendants

[OisinNolan]

See #32

The empty string is being returned by step 2F in cases when an Element that allows name from content has no descendants. Instead, other steps (such as 2I) should be considered. For example:

<input type="checkbox" title="foo">

<input> should have an accessible name of ”foo” here, but accname is returning ””.

This issue is also referenced in #19