Bug#55744 GROUP_CONCAT + CASE + ucs return garbage

Problem: CASE didn't work with a mixture of different character
sets in THEN/ELSE in some cases.
This happened because after character set aggregation
newly created Item_func_conv_charset items corresponding
to THEN/ELSE arguments were not put back to args[] array.

Fix:
put all Item_func_conv_charset back to args[].


  @ mysql-test/include/ctype_numconv.inc
  @ mysql-test/r/ctype_ucs.result
  Adding tests

  @ sql/item_cmpfunc.cc
  Put "agg" back to args[] after character set aggregation.

mysql-test/include/ctype_numconv.inc

@@ -1635,12 +1635,7 @@ CREATE TABLE t1 (a MEDIUMINT NULL) ENGINE=MYISAM;
 INSERT INTO t1 VALUES (1234567);
 SELECT GROUP_CONCAT(IFNULL(a,'')) FROM t1;
 SELECT GROUP_CONCAT(IF(a,a,'')) FROM t1;
-if (`SELECT @@character_set_connection != 'ucs2'`)
-{
-  # Temporarily disable for ucs2
-  # For details, see Bug#55744 GROUP_CONCAT + CASE + ucs return garbage
-  SELECT GROUP_CONCAT(CASE WHEN a THEN a ELSE '' END) FROM t1;
-}
+SELECT GROUP_CONCAT(CASE WHEN a THEN a ELSE '' END) FROM t1;
 --enable_metadata
 SELECT COALESCE(a,'') FROM t1 GROUP BY 1;
 --disable_metadata

mysql-test/r/ctype_ucs.result

@@ -3853,6 +3853,9 @@ GROUP_CONCAT(IFNULL(a,''))
 SELECT GROUP_CONCAT(IF(a,a,'')) FROM t1;
 GROUP_CONCAT(IF(a,a,''))
 1234567
+SELECT GROUP_CONCAT(CASE WHEN a THEN a ELSE '' END) FROM t1;
+GROUP_CONCAT(CASE WHEN a THEN a ELSE '' END)
+1234567
 SELECT COALESCE(a,'') FROM t1 GROUP BY 1;
 Catalog	Database	Table	Table_alias	Column	Column_alias	Type	Length	Max length	Is_null	Flags	Decimals	Charsetnr
 def					COALESCE(a,'')	253	9	7	Y	0	31	8

sql/item_cmpfunc.cc

@@ -3029,6 +3029,14 @@ void Item_func_case::fix_length_and_dec()
   {
     if (agg_arg_charsets_for_string_result(collation, agg, nagg))
       return;
+    /*
+      Copy all THEN and ELSE items back to args[] array.
+      Some of the items might have been changed to Item_func_conv_charset.
+    */
+    for (nagg= 0 ; nagg < ncases / 2 ; nagg++)
+      args[nagg * 2 + 1]= agg[nagg];
+    if (else_expr_num != -1)
+      args[else_expr_num]= agg[nagg++];
   }
   else
     collation.set_numeric();