token middleware - missing grant_type results in unsupported_grant_type error

If the `grant_type` parameter is missing in the request, the token middleware will return with a `501` header with the following body:

```
{
error: 'unsupported_grant_type',
error_description: 'Unsupported grant type: test'
}
```

If I'm not mistaking, [the specs](http://tools.ietf.org/html/rfc6749#section-5.2) state that this should be a `400 - invalid_request` response.

I suggest a patch where the middleware returns a `400` header with the following body:

```
{
error: 'invalid_request',
error_description: 'Missing required field: grant_type'
}
```

Start at the token middleware's handling of a missing grant_type and compare its response with RFC 6749 section 5.2. Verify the status and error body for a request without grant_type; done means it returns 400 with invalid_request and the specified missing-field description.

Written by the indexing model from the issue text.