[css-ruby] White space collapse between ruby annotations

It seems the current spec is not very clear about how white space collapse
should happen on ruby annotations.

There is a sentence says "Where undiscarded white space is collapsible, it
will collapse following the standard white space processing rules", but I
don't find anywhere mentions the normal white spaces inside ruby annotation.

Let's see an example. If we have a ruby tag like:
<ruby>
  <rb>a</rb><rb>b</rb>
  <rt>x␣</rt><rt>␣y</rt>
</ruby>

If we consider the two ruby text boxes are in the same line, and thus use
the standard processing rules to handle the white spaces here, we would get
the same result as:
<ruby>
  <rb>a</rb><rb>b</rb>
  <rt>x␣</rt><rt>y</rt>
</ruby>

But it doesn't seem to make sense having a different result between this
code and
<ruby>
  <rb>a</rb><rt>x␣</rt><rb>b</rb><rt>␣y</rt>
</ruby>

Hence I propose that we do not collapse white space across the boundary of
ruby text box.

- Xidorn