Fixes for issue #870

Shipping Helper:
* Added an isset($dimensions[0][‘dimensions’]) check to avoid a server
warning If the store owner doesn’t enter dimensions for their products
when they’re not using advanced rates.

USPS module:
* Changed PARCEL to STANDARD POST and EXPRESS to PRIORITY EXPRESS to
solve issue #870 and updated all the currently commented (not shown)
USPS services.
* Added a str_replace for the hexadecimal representation of Registered
symbols appearing in the USPS response instead of the decimal
representation. @misulicus reported this, but I don’t know how USPS
decides to send either representation.
* Improved and simplified the validation to distinguish between an
array of arrays or 1-dimension arrays.
* Fixed an asort() warning shown when no quotes are brought back from
USPS.
* Added STANDARD POST and changed EXPRESS to PRIORITY EXPRESS as USPS
default services, if no services where selected at the USPS shipping
admin page.

Any questions or comments are welcomed.