Allow duplicate headers to be appended

Updated the Response object to allow headers to be appended using the standard comma-separated values.  It does not verify that the header can be separated this way, and may cause compatibility issues for older browsers using some headers like Set-Cookie.
Rather than accessing the private "_headers" variable, you can check that Falcon actually returned the proper headers to the WSGI interface like so:

```python
self.assertNotIn('key', self.srmock.headers_dict)
```

FWIW, I realize using self.srmock is not super intuitive, and I am planning to redesign the testing helpers in 0.3 to behave more like the Python requests interface.