Add Set#join spec

library/set/join_spec.rb

@@ -0,0 +1,31 @@
+require_relative '../../spec_helper'
+require 'set'
+
+ruby_version_is "3.0" do
+  describe "Set#join" do
+    it "returns an empty string if the Set is empty" do
+      Set[].join.should == ''
+    end
+
+    it "returns a new string formed by joining elements after conversion" do
+      set = Set[:a, :b, :c]
+      set.join.should == "abc"
+    end
+
+    it "does not separate elements when the passed separator is nil" do
+      set = Set[:a, :b, :c]
+      set.join(nil).should == "abc"
+    end
+
+    it "returns a string formed by concatenating each element separated by the separator" do
+      set = Set[:a, :b, :c]
+      set.join(' | ').should == "a | b | c"
+    end
+
+    it "calls #to_a to convert the Set in to an Array" do
+      set = Set[:a, :b, :c]
+      set.should_receive(:to_a).and_return([:a, :b, :c])
+      set.join.should == "abc"
+    end
+  end
+end