A fix for new conditions being defined and upper condition not updated

mysql-test/r/type_datetime.result:
  result for datetime fix
mysql-test/t/type_datetime.test:
  test for datetime fix
sql/sql_select.cc:
  Fix for a bug caused implicitely by MySQL having to comply with 
  ODBC standard on datetime comparisons.
  
  This failed to update used_tables() for OR condition, which is why
  query returned all rows on query as in .test above, while
  none of types contains NULL nor 0 datetime (last one from ODBC)

mysql-test/r/type_datetime.result

@@ -40,3 +40,4 @@ date	numfacture	expedition
 0000-00-00 00:00:00	1212	0001-00-00 00:00:00
 table	type	possible_keys	key	key_len	ref	rows	Extra
 t1	ref	expedition	expedition	8	const	1	where used
+a	b

mysql-test/t/type_datetime.test

@@ -50,3 +50,8 @@ INSERT INTO t1 (numfacture,expedition) VALUES ('1212','0001-00-00 00:00:00');
 SELECT * FROM t1 WHERE expedition='0001-00-00 00:00:00';
 EXPLAIN SELECT * FROM t1 WHERE expedition='0001-00-00 00:00:00';
 drop table t1;
+create table t1 (a datetime not null, b datetime not null);
+insert into t1 values (now(), now());
+insert into t1 values (now(), now());
+select * from t1 where a is null or b is null;
+drop table t1;

sql/sql_select.cc

@@ -3009,6 +3009,7 @@ remove_eq_conds(COND *cond,Item::cond_result *cond_value)
       == Item_func::COND_AND_FUNC;
     List_iterator<Item> li(*((Item_cond*) cond)->argument_list());
     Item::cond_result tmp_cond_value;
+    bool should_fix_fields=0;
 
     *cond_value=Item::COND_UNDEF;
     Item *item;
@@ -3028,6 +3029,7 @@ remove_eq_conds(COND *cond,Item::cond_result *cond_value)
 	delete item;				// This may be shared
 #endif
 	VOID(li.replace(new_item));
+	should_fix_fields=1;
       }
       if (*cond_value == Item::COND_UNDEF)
 	*cond_value=tmp_cond_value;
@@ -3054,6 +3056,9 @@ remove_eq_conds(COND *cond,Item::cond_result *cond_value)
 	break; /* purecov: deadcode */
       }
     }
+    if (should_fix_fields)
+      cond->fix_fields(current_thd,0);
+
     if (!((Item_cond*) cond)->argument_list()->elements ||
 	*cond_value != Item::COND_OK)
       return (COND*) 0;