String#casecmp no longer raises TypeError

* See https://bugs.ruby-lang.org/issues/13312

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@58839 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

spec/rubyspec/core/string/casecmp_spec.rb

@@ -25,8 +25,16 @@ describe "String#casecmp independent of case" do
     "abc".casecmp(other).should == 0
   end
 
-  it "raises a TypeError if other can't be converted to a string" do
-    lambda { "abc".casecmp(mock('abc')) }.should raise_error(TypeError)
+  ruby_version_is ""..."2.5" do
+    it "raises a TypeError if other can't be converted to a string" do
+      lambda { "abc".casecmp(mock('abc')) }.should raise_error(TypeError)
+    end
+  end
+
+  ruby_version_is "2.5" do
+    it "returns nil if other can't be converted to a string" do
+      "abc".casecmp(mock('abc')).should be_nil
+    end
   end
 
   describe "in UTF-8 mode" do