[WSS-473] - Make it possible to sign the signing BST in SignatureAction

Conflicts:
	src/main/java/org/apache/ws/security/action/SignatureAction.java
	src/test/java/org/apache/ws/security/message/SignedBSTTest.java

git-svn-id: https://svn.apache.org/repos/asf/webservices/wss4j/branches/1_6_x-fixes@1507246 13f79535-47bb-0310-9956-ffa450edef68