JSON support for the Flask test client

[adambyrtek]

This change adds support for JSON requests to the Flask test client, which makes testing JSON APIs much more convenient, and makes the arguments consistent with request.data and request.json.

Before:
response = client.post('/url', data=json.dumps(json_data), content_type='application/json')

After:
response = client.post('/url', json=json_data})

If this gets merged then I have another change to support response.json, which would make it easier to make assertions on JSON responses.

[untitaker]

LGTM, please add a changelog entry and proper documentation.

[adambyrtek]

@untitaker Please take a second look. I've updated the changelog, but couldn't find a good place for documentation. There's no API reference for FlaskClient besides the Werkzeug one. The user guide for testing doesn't cover JSON requests, and I didn't want to completely change it.

[davidism]

Maybe it would be good to extend the json support to Werkzeug's client/response as well. Then the documentation could go there along with what already exists.

[adambyrtek]

@davidism Yeah, that's my next patch, but since this is my first contribution to Flask I decided to submit this piece by piece.

[untitaker]

@adambyrtek I think it only makes sense to do this on one of them -- Flask's test client is a subclass of Werkzeug's.

[adambyrtek]

@untitaker Sorry, I think I misread the comment. Just to clarify, I don't intend to patch Werkzeug (since I believe JSON support is too high-level for that), but I'd like to extend the Flask response object in a separate pull request to allow:

response = client.post('/url', json=json_data})
assert response.json == {'foo': 'bar'}

However, I'm open to any suggestions on how to improve the documentation for this.

[untitaker]

Ahh, I see.

[untitaker]

Ok, currently this is missing documentation -- AFAICT there's no way to discover this functionality from the online docs.

[untitaker]

Otherwise, 👍

[adambyrtek]

@untitaker Sure, I'll try to incorporate this in the testing section.

[adambyrtek]

Moved to #1416.