Merge pull request #1061 from nemerle/RData-type-constification

Changed the RData type field to 'const'

include/mruby/data.h

@@ -19,7 +19,7 @@ typedef struct mrb_data_type {
 struct RData {
   MRB_OBJECT_HEADER;
   struct iv_tbl *iv;
-  mrb_data_type *type;
+  const mrb_data_type *type;
   void *data;
 };