Add specs for String#{eql?,===,==}

spec/ruby/core/string/shared/eql.rb

@@ -11,11 +11,25 @@
     "less".send(@method, "greater").should be_false
   end
 
+  ruby_version_is "1.9" do
+    it "ignores encoding difference of compatible string" do
+      "hello".force_encoding("utf-8").send(@method, "hello".force_encoding("iso-8859-1")).should be_true
+    end
+
+    it "considers encoding difference of incompatible string" do
+      "\xff".force_encoding("utf-8").send(@method, "\xff".force_encoding("iso-8859-1")).should be_false
+    end
+
+    it "considers encoding compatibility" do
+      "hello".force_encoding("utf-8").send(@method, "hello".force_encoding("utf-32le")).should be_false
+    end
+  end
+
   it "ignores subclass differences" do
     a = "hello"
     b = StringSpecs::MyString.new("hello")
 
     a.send(@method, b).should be_true
     b.send(@method, a).should be_true
   end
-end
+end