add support for pre-defined data to .attach

Usually it's easier to not refer to file system and still be able to
attach files to requests and instead of writing

```
request(...).post(...).part().attachment(...).write(...)
```

I want to re-use current .attach API and use it as

```
request(...).post(...).attach('upload', 'my_file.txt', 'Some content')
```

It makes your code to have less lines and be more readable :)