Fixed broken handling of <br>:

- Dropped non-pre container test as it relied on the old local
  useBR param which is now replaced with a global option, so
  there's no easy way to test it on the same page.
- <br> is now replaced with \n unconditionally.
- Literal \n removed on useBR: true.
- Normalized HTML is assigned to a node to get its textContent.
- Merging is attempted only on the original stream is called only
  if there's `<` in the text.
Faster useBR.

Dropped usecase of using <pre> with <br> as unconditional replace
of <br> requires constructing another node and parsing HTML which
is too expensive.