Setting non-multiple select field to null causes misleading error

[mahileeb]

{code}
java.lang.UnsupportedOperationException: You may only deselect all options of a multi-select

at org.openqa.selenium.support.ui.Select.deselectAll(Select.java:219)
at geb.navigator.NonEmptyNavigator.setSelectValue(NonEmptyNavigator.groovy:723)
at geb.navigator.NonEmptyNavigator.setInputValue(NonEmptyNavigator.groovy:689)
at geb.navigator.NonEmptyNavigator.setInputValues_closure40(NonEmptyNavigator.groovy:680)
at groovy.lang.Closure.call(Closure.java:426)
at geb.navigator.NonEmptyNavigator.setInputValues(NonEmptyNavigator.groovy:679)
at geb.navigator.NonEmptyNavigator.propertyMissing(NonEmptyNavigator.groovy:593)

{code}

[mahileeb]

apache/groovy-geb#128