make file path resolution fix more robust

This commit is based on feedback from vitalif on my pull request

https://github.com/mediawiki4intranet/TikaMW/pull/1#issuecomment-24405255

The change involves a static variable that stores the method name
to be used in getting a file system path.  It is computed once
upon first invocation, where we attempt to figure out whether
the File object support the newer API.