This is a compatibility release and has no new significant changes.
v0.8.0 was released with a ruby version restriction of v2.0.0+ but used String#+@ which is only available in ruby v2.3.0+. So this reverts usage of +"" in favour of "".dup which should have the same effect but be more widely compatible and releases a new version with the relaxed ruby version. The next version will require ruby 2.6.0+ again and restore usage of +"".
See #486 for details of the observed problem in environments with older versions of ruby.
(The other route would be yanking v0.8.0 which feels more widely harmful than good.)