by aschobel:

encoding/json.Mashall doesn't encode a field if the tag has a '/' in it
 
type SubscriptiontrackHandlerSettings struct { 
        Enable    int    `json:"enable"` 
        TextHtml  string `json:"text/html"` 
        TextPlain string `json:"text/plain"` 
} 

http://weekly.golang.org/src/pkg/encoding/json/encode.go?s=3491:3534#L415

A JSON object should be able to have a key with a '/' in if you look 
at http://www.json.org/ 

The API in question is SendGrid's: 
http://docs.sendgrid.com/documentation/api/smtp-api/developers-guide/

Discussion
https://groups.google.com/group/golang-nuts/browse_thread/thread/2a806c73457c4638