Calling "getValue" on a multi-select returns nothing if it doesn't have "name" attribute

Usually multi-selects have `name` attribute, which will be used during form submission. This driver users that `name` attribute in a `getValue` method. This renders all other multi-select elements without `name` attribute useless.

In this PR I've actually partially took `getValue` method implementation from MinkSelenium2Driver, where it works just fine.