multipart/form-data request not sending content-type header

I'm trying to test a multipart/form-data endpoint but it seems as though the content-type header isn't being sent when I run abao.

Raml snippet:

```
/stuff:
put:
body:
multipart/form-data:
formParameters:
name:
description: The name of the stuff
required: true
type: string
example: My_Stuff
image:
description: A picture of the stuff
required: true
type: file
responses:
201:
body:
application/json:
```

If I change `multipart/form-data` to `application/json` I can see the request getting further (but then fails because the required input isn't passed... as expected)

Thoughts?

Thanks
Kimo