Fix 'array_merge(): Argument #1 is not an array context.inc:261'

I found that if you specify an 'include' option in a drush alias, the following warnings are returned during a drush rsync:

````
array_merge(): Argument #1 is not an array context.inc:261                                                   [warning]
array_unique() expects parameter 1 to be array, null given context.inc:261                                   [warning]
````

This is a result of $cache being set to bool(false) as a result of a call a little farther up to drush_get_context(DRUSH_INCLUDE). According to http://php.net/manual/en/function.array-merge.php, the recommended method of ensuring both arguments to array_merge() are arrays is to do a typecast, which is what this patch does.