[css-ruby] One pattern is not covered for intra-level white space

There is one pattern not covered by the table for "Interpret intra-level
white space". The pattern is when previous box is ruby base container, and
the next box is ruby base. I think it should be categoried into
inter-segment white space. Then, the last row in that table could be merged
with the first row for inter-segment white space.

- Xidorn