make MockInvocation more benign, returning null for proxy instead of UnsupportedOperationExceptioncore/src/test/java/org/apache/commons/proxy2/util/MockInvocation.java

git-svn-id: https://svn.apache.org/repos/asf/commons/proper/proxy/branches/version-2.0-work@1525262 13f79535-47bb-0310-9956-ffa450edef68