Make inline validation of AT multiple selection widget work

Inline validation of multiple selection widget did not work because either nothing was selected and 'null' was sent as value to at_validate_field or if values were selected, the sent value key was turned into value[] and bunch of warnings like <p><strong>Invalid request</strong></p>  The parameter, <em>value</em>, was omitted from the request.<p> were displayed in the Zope log...

Please review and merge ;-)

Gauthier