Refactoring data integration test base class; adding missing header.
Adding integration tests for MongoDb. Disproves #1081.
Slightly refactor header setting in action request.
Better explain return value from Response::__toString().
Further documentation for response classes.
Adding drain option to action request to allow disabling auto stream reading.

Future versions may disable draining by default.

When the drain option is disabled will not drain stream when stream data was sent.
This reduces memory usage and enables streaming very large files (i.e. uploading
video content). You must read than read stream data manually:

```
$stream = fopen('php://input', 'r');
// Do something with $stream.
fclose($stream);
```

When drain is disabled automatic content decoding is not supported anymore. It must
be manually decoded as follows.

```
// ...
$data = Media::decode($request->type, stream_get_contents($stream));
// ...
```

Adding and expanding tests to document current and new optional behavior.