...even when no parameters are provided (or the provided parameters are `nil`)

The long story:

- #132 changed the behavior for HTTP DELETE requests to behave like GET: put the parameters in the query string, and don't set the Content-Type. This change was merged in d016695
- This broke `rack-test` for certain people, which was highlighted in #200. Arguably, the change incorporated in d016695 was too brutal.
- #212 tried to sort it out, by not setting Content-Type if params are nil, and also reverting the change to put DELETE parameters in the query string. The first part of this (params being nil) caused issues for certain people.

So this PR now tries to once and for all sort this out by:

- Always setting `env['CONTENT_TYPE']`, even when params are `nil`.
- Adding more tests for how `CONTENT_TYPE` and `CONTENT_LENGTH` should behave; some of this does not come from us, but from Rack upstream.
- Settles with the discussion in #220: if you are using `DELETE` and must use query params, put them manually in there like this: `delete '/foo?bar=baz'`. Arguably not very clean, but better than changing back and forth. `params` are overloaded in rack 0.x and will be so in 1.0 also. I am thinking that we should go with the excellent suggestion provided by @malacalypse in #200 to use dedicated `body` and `params` parameters in the long run (and probably use keyword parameters instead, but that's definitely a 2.x feature since it breaks all existing usage.) For now, I think we'll live with the ugliness.

I encourage everyone to give this a try before we merge, to ensure we don't have to revert once more.