[FileBackend] Added support for changing headers on existing objects.

[FileBackend] Added support for changing headers on existing objects.

* Added a 'describe' file operation type to doOperations()/doQuickOperations().
  This can be used by scripts to fill in headers like X-Content-Duration for
  files that already exists.
* Removed wrong comments about removing headers (they don't get removed with null).
* Added some quick unit tests.

Change-Id: I43c5907b59421beaa9487eefac0cdbf8bc6c6d85