regression: exceptions in client callback handler cause callback to be re-invoked

https://github.com/visionmedia/superagent/commit/092e3d6d289f90af2551540a5fbcdf771ca9b7fe commit appears to have introduced an issue:

If the callback invoked after the request completes throws an exception, the callback is invoked a second time. This is particularly painful for testing with something like `supertest` when you want to assert something after an API request. The test assertions failures are caught by `superagent` and the callback is re-invoked (typically not a good idea). The end result is difficult to understand execution.

```javascript
supertest(server)
.post('/range')
.send('some data')
.expect(201)
.end(err => {
assert.isTrue(someThing);
done();
});
```

God forbid you end up in an infinite loop!