Fix behavior of String#to_i/Kernel#Integer to numbers starting with _

#### Before this patch:

  ```ruby
  Integer("_1")  #=> 1
  "_1".to_i      #=> 1
  ```

#### After this patch (same as Ruby):

  ```ruby
  Integer("_1")  #=> ArgumentError
  "_1".to_i      #=> 0
  ```

src/string.c

@@ -2354,7 +2354,7 @@ mrb_str_len_to_inum(mrb_state *mrb, const char *str, mrb_int len, mrb_int base,
     if (*(p - 1) == '0')
       p--;
   }
-  if (p == pend) {
+  if (p == pend || *p == '_') {
     if (badcheck) goto bad;
     return mrb_fixnum_value(0);
   }