Fix usage of Rope#toString in throw_assertion_error

Author: eregon

src/main/java/org/truffleruby/debug/TruffleDebugNodes.java

@@ -410,7 +410,7 @@ public abstract static class ThrowAssertionErrorNode extends CoreMethodArrayArgu
         @Specialization(guards = "strings.isRubyString(message)")
         protected Object throwAssertionError(Object message,
                 @CachedLibrary(limit = "2") RubyStringLibrary strings) {
-            throw new AssertionError(strings.getRope(message));
+            throw new AssertionError(strings.getJavaString(message));
         }
 
     }