Restore this name for transition purposes

Another one used by StringIO. See ruby/stringio#84 and
ruby/stringio#88.

Author: headius

core/src/main/java/org/jruby/RubyString.java

@@ -3535,6 +3535,11 @@ private IRubyObject byteARef(Ruby runtime, IRubyObject idx) {
         return obj;
     }
 
+    @Deprecated
+    public final IRubyObject substr19(Ruby runtime, int beg, int len) {
+        return substrEnc(runtime, beg, len);
+    }
+
     public final IRubyObject substrEnc(Ruby runtime, int beg, int len) {
         if (len < 0) return runtime.getNil();
         int length = value.getRealSize();