GH-125: Support multiple headers with same key

Fixes: GH-125

```
❯ py.test test_httpbin.py -v -k test_response
...
test_httpbin.py:33: HttpbinTestCase.test_response_headers_multi PASSED
test_httpbin.py:27: HttpbinTestCase.test_response_headers_simple PASSED
...
```

```
❯ http 'localhost:8080/response-headers?animal=dog&animal=cat'
HTTP/1.1 200 OK
Access-Control-Allow-Credentials: true
Access-Control-Allow-Origin: *
Animal: dog
Animal: cat
Content-Length: 110
Content-Type: application/json
Date: Sat, 29 Nov 2014 14:20:49 GMT
Server: waitress

{
    "Content-Length": "110",
    "Content-Type": "application/json",
    "animal": [
        "dog",
        "cat"
    ]
}
```