check if something is a buffer

Is there a way to check if something is a buffer?

``` javascript
expect(file).to.be.a('buffer') does not work.
```

Should has this functionality.

``` javascript
You can do file.should.be.instaceof(Buffer)
```

Start at the JavaScript assertion entry point for `.a(...)` and its existing type checks; compare the requested `buffer` form with the `instanceof(Buffer)` workaround shown in the issue. Confirm the intended assertion syntax and add coverage for a Buffer value and a non-Buffer value; done means the requested check works without the workaround.

Written by the indexing model from the issue text.