from2 doesn't work without through2

I try it and get infinite loop in `headStream`

``` javascript
var headStream = from2.obj(function (size, next) {
var file = new File({
path: 'head.js',
contents: new Buffer('head content')
});
next(null, file);
})

series(headStream, otherStream)
.pipe(g.concat(name))
.pipe(gulp.dest(destPath))
```

I try it and get 16 loops (size = 16), but head file doesn't write to output file

``` javascript
var flag = false;
var headStream = from2.obj(function (size, next) {
if (flag) return this.push(null);
...
flag = true;
next(null, file);
})
...
```

If I use through it works!

``` javascript
var headStream = from2.obj(function (size, next) {
...
this.push(file)
next();
})
.pipe(through2.obj(function (file, enc, callback) {
callback(null, file);
}));
...
```

Why? And how to do it without through?

Start with the from2.obj callback and compare the two examples in the issue, especially how the file is emitted and the stream is ended. Trace the ReadableStream behavior around headStream and verify that the single head file reaches the downstream pipeline without through2; reproduce the reported repeated calls before confirming the fix.

Written by the indexing model from the issue text.