[VFS-309] DefaultFileContent will remove thread data on close to avoid leaks

git-svn-id: https://svn.apache.org/repos/asf/commons/proper/vfs/trunk@1644682 13f79535-47bb-0310-9956-ffa450edef68

Conflicts:
	core/src/main/java/org/apache/commons/vfs2/provider/DefaultFileContent.java
	src/changes/changes.xml