PayPal OAuth: 400 Error even after specifying Host

I'm doing this one in golang, and I've been up just about the entire night trying to get it to work. My latest frustration is that I'm getting a 400 error despite the numerous headers I've dropped into my request.

 29     req.SetBasicAuth(app.id, app.secret)
 30     req.Header.Set("Host", "www.sandbox.paypal.com")
 31     req.Header.Set("Accept", "application/json")
 32     req.Header.Set("Accept-Language", "en_US")
 33     req.Header.Set("Content-Type", "application/x-www-form-urlencoded")
 34     req.Header.Set("Connection", "close")

I have auth set up, and every header I could think to add. I've pored over the documentation and have tried the poke it to see if it works approach as well. Heres the information I've been passing in the body with the post request

`{"grant_type": "client_credentials"}`

 42     req, err := http.NewRequest(
 43         "POST", 
 44         app.Endpoint + oauthEndpoint,
 45         strings.NewReader(`{"grant_type": "client_credentials"}`),
 46     )

If you need some more of the code just tell me. But I'm not super sure it'll be a whole lot of good. I wanted to make the post as general as possible as well.

EDIT:

So it seems that despite the fact I am specifying the grant_type in the payload manually it is failing because grant_type is not being recognized

{"error":"invalid_request","error_description":"grant_type is a required parameter"}

Problem was my reader object, it should have been

strings.NewReader("grant_type=client_identifier")

http://golang.org/pkg/net/http/#Header.Set

Header.Set() doesn't add a new header. You want Header.Add(). See the documentation link above.