Add specs for Set#<=> method

library/set/comparison_spec.rb

@@ -0,0 +1,29 @@
+require_relative '../../spec_helper'
+require 'set'
+
+ruby_version_is "3.0" do
+  describe "Set#<=>" do
+    it "returns 0 if the sets are equal" do
+      (Set[] <=> Set[]).should == 0
+      (Set[:a, :b, :c] <=> Set[:a, :b, :c]).should == 0
+    end
+
+    it "returns -1 if the set is a proper subset of the other set" do
+      (Set[] <=> Set[1]).should == -1
+      (Set[1, 2] <=> Set[1, 2, 3]).should == -1
+    end
+
+    it "returns +1 if the set is a proper superset of other set" do
+      (Set[1] <=> Set[]).should == +1
+      (Set[1, 2, 3] <=> Set[1, 2]).should == +1
+    end
+
+    it "returns nil if the set has unique elements" do
+      (Set[1, 2, 3] <=> Set[:a, :b, :c]).should be_nil
+    end
+
+    it "returns nil when the argument is not set-like" do
+      (Set[] <=> false).should be_nil
+    end
+  end
+end