Fix preserve_pattern regex in buffer.rb

Group preserved tag names to reduce false positives.

This is a fairly simple change, but I know this code has changed a lot in master/4.0. The regex matching whitespace sensitive elements didn’t group the names, so only `textarea` needed the leading `<` to match. `pre` and `code` woud match “raw”.

Are there plans to make the same whitespace handling changes to 3-1? If so this commit is probably redundant.