Array#join to accept only strings as sep

src/array.c

@@ -1006,7 +1006,7 @@ mrb_ary_join(mrb_state *mrb, mrb_value ary, mrb_value sep)
 
 /*
  *  call-seq:
- *     ary.join(sep=nil)    -> str
+ *     ary.join(sep="")    -> str
  *
  *  Returns a string created by converting each element of the array to
  *  a string, separated by <i>sep</i>.
@@ -1020,7 +1020,7 @@ mrb_ary_join_m(mrb_state *mrb, mrb_value ary)
 {
   mrb_value sep = mrb_nil_value();
 
-  mrb_get_args(mrb, "|o", &sep);
+  mrb_get_args(mrb, "|S", &sep);
   return mrb_ary_join(mrb, ary, sep);
 }