`include?` returns nil instead of false if not included

[jaynetics]

i found a potentially breaking change compared to the stdlib version of SortedSet:

on Ruby 2.7:

require 'set'
SortedSet.new.include?(1) # => false

on Ruby 3.0:

require 'sorted_set'
SortedSet.new.include?(1) # => nil

RBTree and Set return false in this case, but this code in set leads to nil being returned if anything other than a Hash defaulting to false is used as @hash. I guess this could either be fixed here in sorted_set by overriding #include? and its aliases, or in set by adding !! or so to the return value?

[eneagoe]

Any chance this will be fixed in a future release?

[pmcnano]

I pushed a PR #9