Demonstrate via tests that our api deals with client's accept header

Accepted types: application/json, application/yaml, text/html

In browser, this means that our api is only browsable.

With curl (or other similar tools) however, you can ask with the Accept
headers

For example:

$ curl -v -H 'Accept:application/yaml' -X GET
http://localhost:6543/api/1/revision/edfbd9e840cc82e879d9aae8ecc29c7121ff750
Note: Unnecessary use of -X or --request, GET is already inferred.
*   Trying ::1...
* connect to ::1 port 6543 failed: Connection refused
*   Trying 127.0.0.1...
* Connected to localhost (127.0.0.1) port 6543 (#0)
> GET /api/1/revision/edfbd9e840cc82e879d9aae8ecc29c7121ff750 HTTP/1.1
> Host: localhost:6543
> User-Agent: curl/7.45.0
> Accept:application/yaml
>
* HTTP 1.0, assume close after body
< HTTP/1.0 400 BAD REQUEST
< Content-Type: application/yaml
< Content-Length: 39
< Server: Werkzeug/0.10.4 Python/3.4.3+
< Date: Tue, 24 Nov 2015 13:00:23 GMT
<
{error: invalid checksum query string}
* Closing connection 0

$ curl -v -H 'Accept:application/json' -X GET
http://localhost:6543/api/1/revision/edfbd9e840cc82e879d9aae8ecc29c7121ff750
Note: Unnecessary use of -X or --request, GET is already inferred.
*   Trying ::1...
* connect to ::1 port 6543 failed: Connection refused
*   Trying 127.0.0.1...
* Connected to localhost (127.0.0.1) port 6543 (#0)
> GET /api/1/revision/edfbd9e840cc82e879d9aae8ecc29c7121ff750 HTTP/1.1
> Host: localhost:6543
> User-Agent: curl/7.45.0
> Accept:application/json
>
* HTTP 1.0, assume close after body
< HTTP/1.0 400 BAD REQUEST
< Content-Type: application/json
< Content-Length: 42
< Server: Werkzeug/0.10.4 Python/3.4.3+
< Date: Tue, 24 Nov 2015 13:00:28 GMT
<
* Closing connection 0
{"error": "invalid checksum query string"}%