Object Cache: get_multi refers to nonexistent $to_get variable

[paulschreiber]

In get_multi(), we have:

if ( $to_get ) {
    $vals = $mc->get_multi( $to_get );

The variable $to_get doesn't exist anywhere else in the code.

This is causing an Undefined variable: to_get warning to appear.

[tangrufus]

related: #27

[nickdaugherty]

Fixed by #30