Should merge arrays of table names (+ is for union)

In function drush_sql_build_dump_command

The following operation is wrong (union of arrays):

$skip_tables += $structure_tables;

It should be this instead (merging arrays):

$skip_tables = array_merge ($structure_tables, $skip_tables);

I noticed the problem because the dump size was much larger than expected -- it was first dumping the data of the structure-only tables, then at the end dropping these tables to re-create them without data.