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

Additional to what is mentioned in issue tickets #803 and #683:

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.