Add `then` alias to `yield_self`.
`then' was added in CRuby 2.6.
MODIFY
mrbgems/mruby-kernel-ext/mrblib/kernel.rb
mrbgems/mruby-kernel-ext/mrblib/kernel.rb
module Kernel
# call-seq:
# obj.yield_self {|_obj|...} -> an_object
+ # obj.then {|_obj|...} -> an_object
#
# Yields <i>obj</i> and returns the result.
#
@@ -10,4 +11,5 @@ module Kernel
return to_enum :yield_self unless block
block.call(self)
end
+ alias then yield_self
end

Add `then` alias to `yield_self`.

`then' was added in CRuby 2.6.