added decode() for an rstrip to properly convert it

was getting the following error:

  File "/usr/local/lib/python3.3/site-packages/boto/s3/key.py", line 177, in _get_base64md5
    return binascii.b2a_base64(self.local_hashes['md5']).rstrip('\n')
nose.proxy.TypeError: TypeError: Type str doesn't support the buffer API

which was fixed with the added decode()