[BEANUTILS-456] Recover lost references to write methods.

When querying a write method for a property descriptor the method is
automatically fetched again if its reference was freed. Added a test case
exposing the problem.

git-svn-id: https://svn.apache.org/repos/asf/commons/proper/beanutils/trunk@1555230 13f79535-47bb-0310-9956-ffa450edef68