Delegating consumer endpoints (<service-activator/> etc)
may not have both 'expression' and 'ref' attributes.
Previously, 'expression' was ignored when 'ref' was present.
- Enforce mutual exclusivity in AbstractDelegatingEndpointParser.
- Add tests for illegal combinations.
- Improve error message when 'method' used with <expression/>.
- Change parser to use reader.error() instead of Assert.
- Add element description to all parser errors.
dd6af7b387