PR for issues #803, #683 (Shipping files)

Additional to what is mentioned in those issue tickets:

a) Removed the Google meta conversion (g:) for non existent meta in the
shipping helper. Google doesn’t like receiving meta in the feed which
isn’t part of their meta documentation.

b) In usps_20.php, check if the rates table quoted back is array of
arrays (advanced rates, more than 1 product shipped), instead of a
single dimension array (no advanced rates or only 1 product shipped),
before attempting to merge them using $rate_table =
$this->_merge_arrays( $package_rate_table ). Avoids an error which
loses the rates when it’s a single dimension array, returning an empty
rates table.

Questions are welcomed.