Clear code range when modify/expanding string for IO.read(..,buf).

Fixes #2316.

Test added to MRI in http://mla.n-z.jp/~w3ml/w3ml.cgi/ruby-changes/msg/36766

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

@@ -3144,6 +3144,7 @@ private IRubyObject readNotAll(ThreadContext context, OpenFile myOpenFile, int l
             }
 
             str.setValue(newBuffer);
+            str.clearCodeRange();
             str.setTaint(true);
 
             return str;