[FileBackend] Cleanup behavior for coping/moving a file over itself.

* Previously, for doQuickOperations(), the default move function
  would just delete the file if it was moved over itself. In fact,
  the php-cloudfiles bindings have this same bug in its move function.
* For both copy and move, when the source and destination are the
  same path, make sure that the headers get updated as specified.
  This only applies for the 'overwrite' case (not 'overwriteSame').
* Fixed bad status for doQuickOperations() when copying a file over
  itself using FSFileBackend.
* Clarified the documentation for 'overwriteSame' option.
* Renamed destSameAsSource to overwriteSameCase in FileOp for clarity.

Change-Id: I3f609d9413795c654de27958b1e807b1fc785f31