`application/csp-report` request body should be treated as JSON

:wave:

Some browsers (like the latest Chrome) send Content Security Policy (CSP) violation reports with the content type `application/csp-report`. The request body is always JSON according to [the CSP spec](https://www.w3.org/TR/CSP2/#violation-reports).

When content type is set to `application/json` (explicitly or by default), one may pass an object to `.send(…)` which is automatically serialised to JSON.

I believe it makes sense to extend this behaviour to `application/csp-report` requests.

I ran into this issue when writing a test (using `supertest`) that mimics a Chrome CSP violation report request to our reporting endpoint.

On a related note that perhaps deserves its own issue, the error message is correct, but quite unhelpful:

```
TypeError [ERR_INVALID_ARG_TYPE]: The "string" argument must be one of type string, Buffer, or ArrayBuffer. Received type object

at Test.Object..Request._end (node_modules/superagent/lib/node/index.js:804:84)
at Test.Object..Request.end (node_modules/superagent/lib/node/index.js:777:15)
at Test.Object..Test.end (node_modules/supertest/lib/test.js:125:7)
at node_modules/superagent/lib/request-base.js:238:12
at Test.then (node_modules/superagent/lib/request-base.js:237:31)
```

It would be useful if this error message indicated that the issue might be related to the `Content-Type` being set to something other than the forms of `application/json` supported by the automatic serialisation.

For example, it might read:

> TypeError [ERR_INVALID_ARG_TYPE]: For requests with `Content-Type` set to anything other than `application/json`, the "string" argument must be one of type string, Buffer, or ArrayBuffer. Received type object

… or perhaps something slightly more custom that also prints the received content type. I am not sure if anything other than the `Content-Type` header might affect the automatic serialisation.