Add ability to set 'json spaces' on res.json

[jczaplew]

Currently json spaces can only be set at an application level, but it would be nice to be able to override that setting on the response level. For example, in my situation I'd like the root of an API route to pretty print JSON (app.set('json spaces', 2)), but an actual response to be minified (app.get('json spaces', 2).

I found a way to get around this by setting the json spaces on a response as so:

res
   .set("Content-type", "application/json; charset=utf-8")
   .send(JSON.stringify(data, null, 2));

This works well, but it would be nice to set json spaces directly on res.json in the same way you can set something like Content-type. This PR allows you to do this - for example:

res
   .set("json spaces", 2)
   .json(data);

If json spaces is not set on a given response it falls back to the application's json spaces setting.

[dougwilson]

res.set and res.get is not a bag of keys; you're actually setting a header value because res.set("json spaces", 2) will add the header json-spaces: 2 to your response.

[jczaplew]

Ah! I didn't notice that. Thanks! I'll have to rethink this...

[dougwilson]

No problem :) Feel free to comment here as well: #2422 which has some more discussion, also acknowledging this is an issue we'd like to support :)

[jczaplew]

Great! Thanks! I searched around for a discussion or information on this topic and couldn't find that thread. That helps a lot!

[rlidwka]

New feature added, no tests for it, but code coverage remains the same 100%. Cool. :)