Use array_merge instead of the plus operator so that hooked functions are

Use array_merge instead of the plus operator so that hooked functions are
able to override the ORDER BY condition

This is needed to implement wikiHow's "reverse order" option cleanly,
without any core hacks. Without this changeset, you can hook into
ChangesListSpecialPageQuery hook, but it will be impossible to override
the ORDER BY conditions to get rid of the DESC sort order when the reverse
param is set to 1 in the URL.

For a live example of the feature in question, see
http://www.wikihow.com/Special:RecentChanges (tick the "reverse order"
box, press button and take a look at the results).
For the code behind this feature, see
/extensions/wikihow/hooks/SpecialPagesHooks.php and
/extensions/wikihow/hooks/WikihowHooks.php on the wikiHow codebase.

Change-Id: I2177aed9e4807b90cbde4baf33083da492d3d194