Default the shipping rate only if there is only one rate available.

There were a number of discussions on #1588 that indicated that defaulting in the lowest rate wasn't the agreed behaviour. This PR fixes #1329 by defaulting the shipping choice if there is only one choice available, but not providing a default in all other cases. 

Users who wish to default the lowest can do so using the filters supplied (Or using the plugin I'll push to wp.org shortly!)