Prior to this commit, `str_join` would skip empty items at the start of
the `Iterator` until it found a non-empty item. This contradicts
`SliceConcatExt::join` which doesn't skip anything.
Brought up in:
https://github.com/servo/servo/pull/7776#issuecomment-144317281
I was concerned for a little bit that passing just whitespace to the
split_html_space_chars function would result in len > 0, so I wrote this
small unit test to make sure it does the right thing. Even though it
does do the right thing, I think there's still value in committing the
unit test.
