Add spec for StringIO#set_encoding on a frozen string

* Closes #175.

Author: eregon

library/stringio/external_encoding_spec.rb

@@ -7,4 +7,15 @@
     io.set_encoding Encoding::EUC_JP
     io.external_encoding.should == Encoding::EUC_JP
   end
+
+  ruby_version_is "2.3" do
+    it "does not set the encoding of its buffer string if the string is frozen" do
+      str = "foo".freeze
+      enc = str.encoding
+      io = StringIO.new(str)
+      io.set_encoding Encoding::EUC_JP
+      io.external_encoding.should == Encoding::EUC_JP
+      str.encoding.should == enc
+    end
+  end
 end