if we read a .part file we try to get the file info from the real file

if we read a .part file we try to get the file info from the real file.

fix issue discussed here https://github.com/owncloud/core/pull/11659#issuecomment-60232838

This makes sure that we don't try to calculate the unencrypted size from part files during upload.

cc @DeepDiver1975 @jknockaert please review, thanks!
>  But at this point we no longer operate on the same instance of the cache class like we did when we updated the $partial array

That seems like a bug, making it static will work around it but wont fix the underlying bug.
That may be fixed with #11539 do you have the time to test that?