Don't re-raise as RuntimeException

We expect exceptions from catString, so just re-raise directly.

This caused the ugly internal RuntimeException trace in
ruby/rake#619. This does not fix that issue, but it properly lets
the original Ruby exception propagate.

Author: headius

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

@@ -1662,7 +1662,7 @@ private static void catString(RubyString myString, RubyString str) {
         try {
             RubyString unused = (RubyString) CAT_WITH_CODE_RANGE.invokeExact(myString, str);
         } catch (Throwable t) {
-            throw new RuntimeException(t);
+            Helpers.throwException(t);
         }
     }