Fix string parameter handling in S3Connection.

I was initializing an S3 connection with the following parameter:
```python
calling_format = u'boto.s3.connection.OrdinaryCallingFormat'
```

Which produced this error:
```python
Traceback (most recent call last):
(...)
  File "/lib/python2.7/site-packages/boto-2.19.0-py2.7.egg/boto/s3/connection.py", line 495, in create_bucket
  File "/lib/python2.7/site-packages/boto-2.19.0-py2.7.egg/boto/s3/connection.py", line 533, in make_request
AttributeError: 'unicode' object has no attribute 'build_path_base'
```

To fix this issue, I had to replace my `calling_format` parameter by a pure string:
```python
calling_format = 'boto.s3.connection.OrdinaryCallingFormat'
```

This patch fix string parameter comparison to always get the proper calling format class.