[Dbix-class] Can't modify prefetch

[Ryan Tate]

It appears if you specify prefetch twice on the same resultset, only
the first one sticks. E.g.:

my $rs = $schema->resultset('Artist')->search( {},
                                             {prefetch => ['cds'],}
                                            );
$rs->search({},
           {prefetch => ['stalkers'],}
          );

According to the docs, prefetches are supposed to be merged in this
scenario <http://search.cpan.org/~frew/DBIx-Class-
0.08124/lib/DBIx/Class/ResultSet.pm#Resolving_conditions_and_attributes>

If you monitor the SQL that gets executed, the first prefetch ("cds")
sticks but the second prefetch ("stalkers") doesn't get executed, and
DBIx::Class will go to the database every time you call it on the
fetched items ($artist->stalkers->first).

There's a failing test case here
http://ryantate.com/transfer/dbixc_prefetch.t.txt

And I added an RT entry here
https://rt.cpan.org/Public/Bug/Display.html?id=63360