fix for update_entity (don't need headers kwarg anymore)

update_entity was broken and giving errors like this:

```
Traceback (most recent call last):
  File "/Library/Python/2.6/site-packages/raxmon_cli/common.py", line 125, in run_action
    callback(instance, options, args, done)
  File "/usr/local/bin/raxmon-entities-update", line 47, in callback
    why=options.why)
  File "/Library/Python/2.6/site-packages/rackspace_monitoring/drivers/rackspace.py", line 719, in update_entity
    headers=kwargs.get('headers', {}))
TypeError: _update() got an unexpected keyword argument 'headers'
```

It looks like this may have been a residual headers kwarg from last commit...