Bring obj server unit tests to > 98%

This set of changes attempts to bring the unit test coverage to over
98% for the object server module.

Two changes to the object server are made with this patch:

1. The try/except block around diskfile.write_metadata() was removed
   at the end of the POST method

The write_metadata() method of the DiskFile module does not raise
either the DiskFileNotExist or DiskFileQuarantined exceptions on that
code path.

2. The conditional container_update() call was removed at the end of
   the PUT method

The container_update() calls is performed when a new object is created
or when an exist object is updated. Since we already report old
timestamps as 409s (Conflict) we always perform the update.

We also fix an existing test to clear the hash prefix so that it can
actually detect the async pending pickle file creation for a failure
mode.

Change-Id: I71ec9dcf7c0ac86e56aa0f06993d501fdfa22d5b
Merge "Bring obj server unit tests to > 98%"