FilesystemImagePersistenceStrategy: fix path to the file which prevents it to be saved to the disk.

Before MultipartFile.transferTo() tries to save the file relatively to directory from
multipart-config/location (in web.xml). As result instead of put file to /data/uploads it tries to
write it to /tmp/data/uploads and because intermediate directories doesn't exist it silently exits.

See also my question at SO about spy objects:
http://stackoverflow.com/questions/21562276/how-to-verify-argument-of-method-of-spying-object

Addressed to #132