elements should not allow both 'ref' and 'expression'

The following configuration does not trigger a parsing error, but then can lead to confusing results since the expression is ignored and a method matches following the default rules as if only 'ref' is provided with no 'method' attribute:

<int:service-activator input-channel="input" ref="someBean" expression="@someBean.someMethod(payload)" output-channel="output"/>