Modify string even if it didn't expand

String data might be shared, so modify to ensure we have a writable
version.

ext/java/org/jruby/ext/stringio/StringIO.java

@@ -1559,6 +1559,7 @@ private void ungetbyteCommon(ThreadContext context, byte[] ungetBytes, int cp, i
                     strBytelist = str.getByteList();
                     strBytelist.setRealSize(len + rest);
                 } else {
+                    str.modify();
                     strBytelist = str.getByteList();
                 }
                 strBytes = strBytelist.unsafeBytes();