OAuth1 not working when using SIGNATURE_TYPE_BODY

If you use <code>oauthlib.oauth1.SIGNATURE_TYPE_BODY</code> as your OAuth signature type the plugin doesn't update the <code>req.body</code> value it instead keeps the 'new' body inside <code>req.data</code> which is then never merged with the  <code>req.body</code> and therefore the OAuth params are never transmitted. 

Directly replacing the <code>req.body</code> with the result of the sign request works here. requests.models.prepare_auth takes care of recalculating the 'Content-Length'-header.