S3Client.head_object is broken

[mithrandi]

  File "/home/mithrandi/code/txaws/txaws/s3/client.py", line 495, in <lambda>  
    d.addCallback(lambda (response, body): _to_dict(response.responseHeaders)) 
exceptions.AttributeError: 'Response' object has no attribute 'responseHeaders'

The attribute is named headers, not response_headers. Sadly there is no test coverage at all of this code path, so the test suite doesn't catch this.