Bug: Append to wgAutoloadClasses

Bug: Append to wgAutoloadClasses

When generating $wgAutoloadClasses for an extension the generator
currently outputs an assignment (=), but it is unlikely this is
the desired result. An extension wants to append to the existing
$wgAutoloadClasses. This bug is an unintended consequence of I75403ace
which changed the generator from assigning one key per line to using
an array literal.

This patch changes the output only when generating $wgAutoloadClasses
to the += operator which adds to the array any value that is not
already in the array.

Change-Id: I7d42ee5dc829991c6562878f0c90a06fadb1b6a6