Translation bugged for 'mu-plugins' installation

Hi.

Translation doesn't work for 'mu-plugins' installation because the URL of the MO file is passed to the 'load_textdomain' function instead of its path.

Line 698:

$domainmap_dir = trailingslashit(WPMU_PLUGIN_URL);

Should be:

$domainmap_dir = trailingslashit(WPMU_PLUGIN_DIR);

Thanks.